Have you watched the second of the two free lectures on control accounts, because I go through this example and explain about refunds? (You should not use the notes without the lectures – it is in the lectures where we explain and expand on the notes.)
A refund is simply a repayment of cash. When a customer pays us cash the entry is Dr Cash Cr Receivables. If we repay (refund) cash for any reason (such as, for example, they overpaid) then it is the reverse – Dr Receivables Cr Cash