The poritz was the nobleman. They were the feudal landowners, the russian aristocracy. They often lived in France, since that was fashionable. They needed someone to run their estate and collect the rents, and would often hire a Jew.
This caused grief from both ends, as the landowners would be upset if the profits were not coming in, and the ???? hated them for collecting the money.
The Cossack rebellion of 1648 of was rebellion against these landowners, and was joined by the peasants. Naturally, the Jews were targeted, the landowners were largely not even there.