You make an initial invoice to your client in the amount either evident in the price lists or coming from the agreement you two signed off. Be the scenario what it is, at a later date, your client discovers that he or she would love a discount in a more considerable amount you didn’t initially agree on. Now, in case you do agree that you’re going to give your client the discount, you should also recognize for this additional rebate given. With the accounting entry to follow what you essentially do, is increase either the expense on the discount account on your income statement or decrease net sales and also decrease the accounts receivable amount (presuming your client has not yet paid for the goods or services they received):

Db: Sales or the discount account recognized separately under the Net sales
Cr: Accounts receivable

The same accounting entry more or less should be used in situations where the initial invoice was wrongly made (i.e., prices, amounts or goods were incorrect). What should be done in the accounts in such cases, is a simple credit to sales (not using the discount account though) and also decrease in receivable balances. You’re specifically adjusting your sales accounts and relating receivables so they would reflect the proper sales amount.