Credit note for free of cost as per agreement with debtors

Dear Sir ,
We issue the monthly credit note against supply in place of goods and price difference and adjust with invoices on same month
Before Gst implement in our country we issue credit and debit the output vat , can we practice same method in gst invoices and we claim as itc these all things foc as per agreement between company ,distributor and receiver of goods
please help us

with regards
Arshad