A voucher or gift certificate was 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, something he later admitted in a recorded confession. (TAS: "Mudd's Passion")
After William T. Riker won more latinum at the dabo table in Quark's than Quark could pay, Quark gave Riker vouchers worth twelve bars of latinum vouchers which were only redeemable at the bar. In 2370, Riker offered them back to Quark in exchange for information about Lursa and B'Etor, only to be told afterwards that Quark had voided them while they had been talking. (TNG: "Firstborn")
Brad Boimler was offered a gift certificate for use in the gift shop at Quark's, which he accepted instead of half that value in latinum. (LD: "Hear All, Trust Nothing")