You have a lot of loose ends. You have an unpaid debt that is in collection. While you have been making payments against this debt, you apparently have no written contract with them specifying their agreement to a payment plan. So, potentially, even if you continue your current payments, they may be able to decide to demand full payment now.
In my opinion, the very first thing I would do is to know the date and period of running of your state statute of limitations. This is important, should they at any time bring legal action for collection of the debt. Statute of limitations on collection of debt varies under the civil code of each state. Most states begin running of your SOL based on the date of "first cause of action," which is normally the date of first delinquency (DOFD) on the OC account. Most state SOL periods run for from 3-5 years.
Some states provide for reset of your SOL based on partial payments, or firm offers to pay.
You could try to negotiate a written payment agreement with them, thus fixing your known monthly obligation, but this mightt spur them to reevaluate your current, and strictly oral, "agreement" with them.
I cant offer firm advice other than to suggest that you first be fully aware of their legal ability to collect by knowing the expiration of your state SOL on this debt.
