My two cents...
I bill for 3 different MAC's, and all of them have the same process for this situation. (Your MAC may be different, but I am not holding out any hope). You need to go into their portal (which you have to be registered for), and do a Claim Correction for POS. Once they receive that, they will send a letter asking you to refund what they already paid you, and once they get that refund, they will reprocess the claim for payment at the correct amount.
It may be possible to do this all on paper if you aren't registered with the portal. You will have to find the proper forms on their website.
I have never known Medicare to just pay more on a claim/ask for a refund for partial over-payment before. They are strict with their accounting practices.