Le guy who went to jail is dumb as fuck.

Common prison currency is either postage stamps or foil packs of mackerel

Source:https://federalcriminaldefenseattorney.com/prison-life/special-tactics/the-black-market-in-federal-prison/#:~:text=Money%20is%20prohibited%20in%20federal,for%20%241%20from%20the%20commissary.

Another favorite is cigarettes