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If they can prove the amount of overpayment then you do owe them the money. You are entitled to proof of the overpayment and proof of the checks they wrote to you which are in their control to provide. Thus, you need to send them a letter informing them that you are not refusing to pay, but before you agree to anything you are demanding they provide you proof that they did overpay you, which you are entitled to.
I truly aim to please you as a customer, but please keep in mind that I do not know what you already know or don't know, or with what you need help, unless you tell me. Please consider that I am answering the question or question that is posed in your posting based upon my reading of your post and sometimes misunderstandings can occur. If I did not answer the question you thought you were asking, please respond with the specific question you wanted answered.
Kindly remember the ONLY WAY experts receive any credit at all for spending time with customers is if you click on OK, GOOD or EXCELLENT SERVICE even though you have made a deposit or are a subscription customer. YOU MUST COMPLETE THE RATING FOR THE EXPERT TO RECEIVE ANY CREDIT, if not the site keeps your money on deposit.
Also remember, sometimes the law does not support what we want it to support, but that is not the fault of the person answering the question, so please be courteous.