Who currently owns the debt?
Stated differently, does the debt collector only have assigned collection authority, and thus the original creditor still owns the debt, or has the debt collector purchased the debt?
If the debt has been sold, the original creditor can no longer accept any payment, as you no longer have any debt due to them.
You thus can no longer make a PFD offer to the creditor, and can only now make payment/settlement negotiations with the debt collector.
If the creditor still owns the debt, they can still require you to deal only with their assigned debt collector.
It is up to the creditor how or whether they wish to deal directly with you.
Note that if you do reach a payment agreement with the creditor, the creditor cannot make a PFD agreement that includes deletion of anyu reported collection. Thus, if your primary concern is remova; of the collection, then if you get a PFD from them, you have an assured removal of the collection.