Not to nitpick, but the title of your thread is misleading. This is about credit reversal. You have been paid for the assignment from what I understand.
I have been in this position a few times and can offer this:
#1; Understand that that sometimes things go awry and the MSC is not always in control of the situation. If they are communicative with you, work with them to allow them a resolution within a reasonable amount of time. I personally feel like 90 days to handle a financial mistake is enough. Check with them weekly to make sure they are aware of the situation.
#2; Once the time has passed, advise management that your agreement with them was not upheld from their end, and ask that they personally reimburse the charges, with the agreement that you will return the reimbursement if the charge eventually reversed. Make sure that you document any interest charges incurred and have them reimburse those as well.
This approach has always been successful for me. Handling the situation calmly and professionally will most likely improve your reputation with the MSC if you want to continue working with them.