Pro Forma Invoice

Hello

In February we received and paid a proforma invoice,  we then reconciled the bank and did a vat return which included the transaction.  We have now finally recieved the actual invoice and it is for less which gives us a credit with the supplier.

How do I correct this in Sage - I don't think I can delete or amend  the pro forma invoice due to it being reconciled.

My first thought is to post a credit for the inital amount?

Thanks

Lynn