I am assuming that the debt collector now owns the debt, as if the OC still owns it, technically any additional interest is accrued to them, not the debt collector, and the debt collector is simply attempting to collect interest asserted by the OC. If the debt collector has purchased the debt, they become the owner, and thus subject to whatever provisions of law govern their charges.
FDCPA 808(1) makes it a violation for a debt collector to attempt to collect any amount that is either not specfically authorized in the contract that created the debt, or is otherwise permittted by law.
I would presume that the OC did not include any provision for interest in their initial contract that violated any usery laws. If they did, that contract can be challenged.
Any additional amount tacked on by a debt collector becomes a violation of the FDCPA if not permitted by law.
Absence of a statutory authorization for any such amount would, in and of itself, not be permitted by law. The addition of a usery law that that explicitly prevents such amounts would be the icing on the cake.
The key, in my opinion, is what is authorized in the original account agreement, and whether that rate is in compliance with any usery laws.
