Conflicting Balance

I currently have a debt with Sears. The account has been charged off and I have been paying to a collection agency since 2007 with an arrangement. The other day I received a call from a different collection agency regarding the same debt. I explained that I already had arrangements with a different agency. They claim that they now have the account. I confirmed this with PRS, which was the agency that I was paying to. I called back Centrel to set up an arrangement with them. I asked them to verify the balance due and they are trying to tell me I owe $8150 instead of the $6542 the PRS showed as the balance. I called Resurgent (they actually owe the debt) and they are telling me it is because of interest. PRS is telling me that they were never told to add interest to the account, so that is why it doesn't reflect on the balance. The guy from PRS was actually very nice and told me not to pay on the $8150 balance because it should only be $6542. He also told me to request a copy of my original contract when I signed up for the Sears card to see if there is anything in there that says they can charge interest if moved from one agency to the next. He said unless it says that, I should not be charged interest for the account. He also said to have them put in writing why the balance is more and a detalied listing of the "interest" charges. I am just not sure where to go from here. Please HELP!!!