He makes an appearance on DS9's predecessor The Next Generation in the episode "Firstborn." Here, he's hailed by Commander Riker to give information about the whereabouts of the Duras sisters. Quark brings up the time Riker won at the game Triple Down Dabo at one of his tables. Riker reminds Quark he's still owed 12 bars of latinum for his win and was given vouchers in their place, vouchers only good at Quark's Bar. Too bad Quark's brother misplaced the safe key.
So they make a bargain---information about the Duras sisters in exchange for the return of those vouchers.