How to find out where a credit for an overpayment originated

I recently took over an accounting position, and the standard practice at the company has been to apply credits for customers' overpayments against their oldest open invoice.  Because they would only notify the customer after applying the credit, a customer has ended up consistently overpaying because they are not paying attention to the emails from us about their available credits.  To rectify this I have been trying to go back and apply the payments according to their direction, and leave any remaining credits open until they direct me how to use them.  Because these credits had already been used, some of them have been transferred between jobs using general journal entries in the Account for Credit Transfer clearing account.  The customer wants to know where these available credits came from.  Because they were all washed through the clearing account I can't see what payment or credit they originally came from.  

My questions is how do I figure out where these credits originated from, given that they've all been washed through the account for credit transfer and now appear as General Journal entries?