Small Claims Collection

What is the name of your state? What is the name of your state? Washington, DC
If an agreement has been made with a collection agency, what is the law as to the collection charging a monthly interest until bill is paid? I made initial payment of $250 as a agreed on a $900 bill and $150 was applied to interest.

Simply, if your agreement didn't mention interest or how it was to be applied, the lender (or his agent) has the right to charge interest and apply it as noted in your original lender agreement.

And the application of interest first from the payment and the balance to principal is common.