Guest, have you contacted the original creditor, won't they accept payment from you?
However, if you go with collection agency, always ask for validation and clearly tell them that you will not pay without proper paper work. Once you have the documentation, check it and dispute any discrepancy over there.
While paying to them, make sure they agree to report your account as paid or paid as agreed. This agreement should be in writing too.
Do you have the green slip signed by them? You can use it as evidence. If you request for validation, they cannot continue with collection unless they validate the debt.