Foreign Money Is Proof of Guilt

A crime, usually an international or diplomatic one, has happened. Someone was paid off by a foreign power. And how were they caught? The foreign power was inconsiderate enough to pay them in their own, rather than domestic, currency. Sometimes this can be justified, for example the bribing party may not have much hard foreign money to hand or the bribed party may have intended to flee the country and defect, but often it is not.

Truth in Television in that possessing foreign currency was itself a crime in the USSR and some of its satellite countries. In fact, it is a crime in a lot of countries. It's called "currency smuggling,"although you generally have to have quite a LOT of foreign money to run afoul of it. If you're on vacation and bring home a few coins to show your family, you should be OK.

Bit of a Undead Horse Trope in that it's much better proof in the past, when rulers coined their own money—both Marc Antony and Octavian issued their own currency, for example, although they shared rulership of the Roman Empire at the time.

It's also not uncommon for somebody to have foreign money planted in their possessions to make it look like they did it. Sometimes part of a False-Flag Operation. Can overlap with Global Currency Exception.

Film

 * In 300, Theron is exposed when he is caught with Persian coins. (In Real Life, Persian money was made from precious metals but Spartan money was made from iron; metaphor alert!)
 * Actually, he is already dead by that point, but it does serve to convince the assembly that Leonidas was right.
 * Sparta didn't mint coins in the Classical period; their currency remained long spits of iron to discourage hoarding wealth, so any coinage was foreign.
 * The Girl in Gold Boots, a Mystery Science Theater 3000-riffed film from the 60's, has Critter introduce himself by asking for change for a hundred, then a fifty. Buzz then tries to rob him, inverting the trope. Later on, we learn that said money is actually Nepalese money.
 * Chain Reaction. The protagonists have money planted in their homes to make it look like the Chinese government are bribing them.

Literature

 * In Jingo the apparent assassin of the visiting prince is apparently paid in Klatchian currency.
 * In the X Wing Series, an evil scientist is captured by the New Republic and bargains with them, hoping to spill secrets in return for amnesty, anonymity, and half a million credits. They're the good guys, so she gets it, but the man dealing with her is a Twi'lek who deliberately infuriates her. When he takes her to Coruscant just prior to release she insists repeatedly on Imperial credits, not New Republic ones. The exchange rate's not mentioned, but she does have a permit to carry that much. Then Coruscant security searches her, finds half a million Imperial credits, and arrests her, because that is illegal. The Twi'lek (who's also a trained lawyer) happily informs her that because she was caught trying to smuggle it into the capital the likely charge will be sedition and a mandatory life sentence, neatly preventing her from walking off and selling enemy her evil science skills under a new name. Given the fact that half a million credits is a shit load of money (According to the RPGs this is enough to buy a private, custom yacht and a new freighter) and Coruscant had just recently been taken by the New Republic this is a fairly legitimate crime. The Twi'lek did give her plenty of opportunity to change her mind, though.
 * In The Master and Margarita there was a guy who got arrested because the police found foreign currency in his apartment; he had accepted a bribe from Satan, but in roubles. Woland/Satan then anonymously called the police, who found the currency, now mysteriously American dollars.
 * This came up a number of times in the Belgariad saga by David Eddings. In this case, however, it is not the stamp on the coins that is a giveaway, but the color—the Exclusively Evil Murgos have access to a virtually bottomless gold-mine, which has some sort of contamination in it, giving the gold a red tinge. And they are quite free with it... as such, anyone caught with red gold in their pockets has probably been bribed by the Murgos.
 * Murgo gold is also supernaturally addictive, which explains both why they're so lavish with it and why possession of it is considered damning evidence rather than merely suspicious by the main characters.
 * Rogue's Home by Hilari Bell has a variation.
 * In Cold Copper Tears, Garrett finds several temple-minted coins that identify the Sons of Hammon cult as the bad guys. Not only are these coins clues, but they're also illegal in Karenta, where the cult and its coinage were outlawed by royal decree over a century before. Justified in that the cult's leaders are so brainwashed that they think no mortal agency can stop them, and that the end of the world is imminent so it won't matter if they're exposed.
 * In A Feast for Crows, Cersei Lannister discovers one of the jailers who was guarding Tyrion had some gold pieces that might have come from House Tyrell, who were trying to marry their daughter to her son. Since Tyrion had just  this made her suspect the Tyrell's had bribed the jailer to free Tyrion, even though Lord Tyrell wanted Tyrion dead since his daughter could have died at his hands.
 * Variation: In The Day Watch, Vitaly discovers that his bag is full of dollars and immediately thinks he's up to something illegal.
 * Averted at least once in Robert Jordan's Wheel of Time series. After a man who tried to kill Rand is found to have Tar Valon coins one of the Asha'man  argues that this is evidence that the Aes Sedai planned it. Cool Old Guy Davram Bashere promptly points out that many men in the area, including Davram himself, have more than a bit of Tar Valon currency.
 * In book two of the Knight and Rogue Series Fisk begins to suspect is the criminal when he sees that one of the victims had been paid off with money from very far north, farther than most traders go, but his newest suspect has been bragging about trading as far as.

Live-Action TV

 * In Babylon 5 Garibaldi is framed for a bombing, and as part of this Centauri Ducats are planted in his quarters. Somewhat justified in that Ducats seem to be the hard currency of choice for many races.
 * In Adam-12, a soldier at the Los Angeles airport coming home from Japan claims to have been robbed. The alleged pickpocket says it's his money. The cop, looking at it, points out that Japanese Yen spends pretty hard here.
 * In the BBC series Sherlock, Sherlock Holmes suspects a car dealer of having lied about traveling overseas. He sneaks a peak in the man's wallet and sees a Colombian banknote, the final clue he needs to solve the case.
 * In Castle, a victim is suspected to be a spy involved in something highly questionable when his car is discovered with a large quantity of Euros in the trunk.
 * When Phoebe was a Salvation Army bell ringer during the holidays on Friends she was disillusioned by her customers and was rude back and her corner was taken away. When the new girl came to take over Phoebe said she would give her one piece of advice: to "watch out for that bitch" (a lady who tried to donate a Canadian coin).

Tabletop Games

 * Shadowrun came up with the idea of having the company that hired the shadowrunners pay them in corporate scrip or company stock instead of real money. There are legitimate reasons to do this. However, they apparently didn't consider that if the shadowrunners were caught after being paid, their having the company's scrip or stock would be a dead giveaway that that company was behind the shadowrun.
 * That is, assuming the stock is actually from the company that is behind the run, not the one that your real employer wants you to think that it is from, unless...
 * For that matter, most forms of hard currency (as opposed to electronic credit or corporate scrip) are illegal in most of the Sixth World, although that doesn't stop people from trading it on the black market.
 * In the card game Banana Republic, the objective is to pay off enough electors to become President of Banania. Now, the citizens of the Republic are used to corruption, but they're also very patriotic; if some muckraking journalist discovers that an elector has been paid with evil American dollars, as opposed to nutritious home-grown bananas, the elector will be forced to repudiate the bribe.

Western Animation

 * In one episode of Biker Mice From Mars, Lawrence Limburger once tried to rid himself of the mice by framing them. The final evidence against the officer bribed by Limburger was a huge pile of Plutarkian money found on the officer's possession.

Real Life

 * Harpo Marx was suspected of being a spy when he entered the USSR because he had rubles. But he had simply exchanged his dollars for rubles with another passenger on the train, not knowing that he had to do it through official channels. So in his case domestic money was proof of guilt.
 * Actually, USSR had state monopoly on currency exchange, so during darker times possessing foreign currency could be considered evidence of some crime.
 * Former Irish Taoiseach (Prime Minister) Bertie Ahern made some unusual deposits to his bank account when he was finance minister—he claimed that he received IR£16,500 from friends as a loan, and about GB£8,000 as a whipround (contributions collected from a group of people for a specific purpose) when he was short on money after his separation. But the sum IR£24,838.49 converts exactly to a GB£25,000 lodgment (deposit).
 * On another occasion, a IR£28,772.90 lodgment was made, which he claimed represented about GB£30,0000 in cash. But this amount converts exactly to US$45,000. Clearly something odd was going on, but as yet no charges have been laid.
 * Though not using foreign money, a man accused of robbing several vending machines paid his bail with quarters, dimes, and nickels.
 * This trope collided with Poor Communication Kills during U.S. Special Forces "Robin Sage" exercise in 2002, a month long war game/ final test for graduating Green Berets where several counties of North Carolina become "The People's Republic of Pineland", complete with police and local government participation. During a traffic stop, a participating Lieutenant and Sergeant attempted to bribe the Sheriff's deputy with "Pineland Don", the official Monopoly-esque currency of the exercise, not realising that the deputy had not been briefed. This led to an escalating series of events culminating in the Lt. getting a full can of mace to the face and a fatal gunshot wound, and the Sgt. taking two non-fatal shots in the back while attempting to run to cover with his blank-firing machine gun.
 * There is an urban legend of an influential Soviet man who made a lot of money illegally and converted it to US dollars. However, thanks to his influence, the KGB couldn't even search his house for evidence. He was about to emigrate to the US, and invited many influential friends to his going-away party, including the American ambassador. After the party, he boarded the plane and left. The KGB search his luggage and find no money. They then search his house and find a large pile of ash. Turns out he had the ambassador witness him burning all his cash and sign an official document that a certain amount of currency was indeed destroyed. All the man had to do was go to the nearest Federal Reserve bank in the States and show them the document to get his money back (it is US government practice to replace provably destroyed currency.)