A/R Credit Memo

Hello Experts,

Just want to discuss one instance.

Suppose 5 QTY for Rs. 100 Each

Created SO - Delivery - A/R Invoice (Rs. 500)

Now when customer got the invoice they corrected it by Rs. 90 for each item.. now their is a difference of Rs. 50

IF i am creating A/R credit note then my gross profit is affecting. please tell me how to correct it and also at what situation we give credit Note ??

After getting the payment from client or it can be done before also ?

Please reply