(written from a Production point of view)
According to the script of "Dramatis Personae", pistres were a type of currency. However, the scene in which pistres were mentioned did not make it in the episode as aired. The script specified the pronunciation to be "PISS-trees".
In the cut scene, Quark was furious to receive only two pistres from a customer. They were described as tiny coins, with emphasis on "tiny".