Voucher

A voucher is a unit of exchange.

On Ophiucus VI, Harcourt Mudd once used fake Federation vouchers to fool two miners out of a year's supply of dilithium crystals. 

After William T. Riker broke the bank at Quark's Bar by winning more latinum at the dabo table than Quark could pay, Quark offered Riker vouchers that were only redeemable at the bar. Quark apparently set up a house account for Riker, and debited the vouchers after Riker offered them back to Quark in exchange for information about Lursa and B'Etor.