Credit memo request to correct Sales order after customer payment

Hello all,

Could you please give me any suggetion on this issue:

We uploded sales orders  from legacy system into sap and create invoice but we made a mistake, we didn't take into account a discount in price conditions.

The Customer payment is correct and also uploaded from legacy system.

I'm required to correct the difference by doing a Credit memo request and credit memo with reference to sales sorder, so my questions :

Do I create credit memo request with ref. to SO for all items, quantities and price or just for a discount amount (what i think) ?

Any input will be appreciated.

Thanks.