Can agencies revise resettlement payments after you've already finished paying off on a verbally agreed sum?

Verbally is NEVER a good idea. Hopefully you have written proof. If so, you can alway submit it to the Credit Bureau. At the very least, you can send in a comment stating you paid in full "per your verbal agreement". Then any Creditors can see that info when they pull your report. If you don't have written proof, you may be out of luck. Try to NEVER do anything verbally. Not all of us are "true to our word".