Recovering Excessive Funds for Unused and Needless Drugs Act of 2021 or the REFUND Act of 2021 This bill requires drug manufacturers to issue rebates to the Centers for Medicare & Medicaid Services (CMS) in relation to discarded amounts (i.e., amounts remaining after administration) of single-dose vial drugs that are covered under Medicare. Manufacturers that fail to comply are subject to civil penalties. The CMS must determine rebate amounts based on payment claims from providers. (Currently, providers may receive payment under Medicare for discarded amounts of single-dose vial drugs through the use of a specific claims modifier.)