[chemeng]

Typically payments in the foreign currency are pegged to exchange rates, in which case you’re still paying the same amount of “value” whether in pesos or dollars.

Regardless you’d be subject to Section 988 and could potentially owe capital gains (or ordinary income) tax if you had a large enough gain from buying, holding and then exchanging the foreign currency.

The US code treats all non-USD currency as property, so you can (mostly) think of it as any other transaction with an exchange between assets. There are some nuances and exceptions of course.