An overpayment letter is a formal request to repay a debt owed to the Medicare Trust Fund. Payment is due upon receipt of the notice. Send the payment with a copy of the overpayment letter received or request an immediate offset.  Interest will accrue 30 days from the date on the overpayment letter and every thirty days thereafter. On day 40, we will immediately begin offsetting and claim payments will be withheld and applied until the entire debt is collected.

The letter number or AR number must be included with the payment in order to apply the refund properly, without delay.

Reference

https://www.cms.gov/Regulations-and-Guidance/Guidance/Manuals/downloads/fin106c03.pdf