You cannot unapply a payment when you transfer the payment to the history tables in Receivables Management in Microsoft Dynamics GP
  Problem :       When you transfer a payment to the history tables in Receivables Management in Microsoft Dynamics GP or in Microsoft Business Solutions - Great Plains, you cannot un apply the payment.       To work around this problem, use one of the following methods.       Method 1: Remove the payment and the applied documents from the history tables   1.      Use the Remove Receivables Transaction History window to remove the  payment and applied documents. To do this, use one of the following methods:   o     In Microsoft Dynamics GP 10.0, point to Tools  on the Microsoft Dynamics GP  menu, point to Tools , point to Sales , and then click Remove Transaction History .   o     In Microsoft Dynamics GP 9.0 and in Microsoft Business Solutions - Great Plains 8.0, point to Utilities  on the Tools  menu, point to Sales , and then click Remove Transaction History .   2.      Reenter the payment and the ap...