In barter... Is it "to pay in something" or "to pay with something"?

Solution 1:

"In" if the items used for payment are widely understood to be a token of exchange, similar to currency

The desk clerk presented me the bill which listed various currencies. I paid in US dollars.

They wanted 200 Swiss Francs. I paid in Euros.

The farmer didn't have cash to pay his neighbour to repair their shared fence, so he paid in chickens.

"With" if the items used for payment are not widely used as a token of exchange

The farmer tried to pay for his language lesson with chickens. "Quelle horreur!", exclaimed the startled lady, shooing them out of her house.