Thank You. There was no previous contract nor specific agreement, there was just one promissory note.
A lawyer should review the promissory note
State law likely does apply not federal unless there is diversity between the two parties i.e you are in different states, or the amount of the note is over $75K....
We are in different States...I didn't think it would matter though. Amount of promissory note has been already fully paid. They re-billed me with solely a collection fee for $900 on original debt of $2500.
The promissory note must say that it can charge late fee's or interest for you to have paid more than the original note. Or had a provision that you pay their collections or costs of collections etc. If not you can send a letter and demand an accounting and/or then sue them on the accounting asking for what you overpaid plus attorney fee's.