Review the contract very carefully, sentence by sentence. If the contract says nothing about charging you interest, then the collection agency cannot charge you interest.
Send the collection agency a letter (a) stating the you dispute the validity of the debt; (b) requesting verification of the debt; and (c) requesting an account history of the debt. You are entitled to this information under the Fair Debt Collection Practices Act (federal law). When you receive the response from the collection agency, review it carefully. If there are interest is included in the calculation of the current balance due, then you should dispute those charges IN WRITING. When you eventually agree with the calculation of the current balance due, then you negotiate IN WRITING payment or settlement of the debt.
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