There are a number of ways vendor discounts can be added, including adding to individual invoices.
1.Add a negative expense PO for the amount of the discount and select a GL account for vendor discounts. This will generate a credit to be applied to the invoice but will not reduce the COGS on the original PO.
2.Add a negative expense PO for the amount of the discount and link it back to the original PO. This will reduce the COGS of the stock on the original PO on a pro rata basis and won't report into a separate GL account. This will generate a credit to be applied to the invoice.
3.To add to individual invoices, add a journal stock code for vendor discount, then add this stock to each PO for the vendor. Order and receive a quantity of 1 and enter the discount amount as a negative value. This will reduce the PO value so it matches the vendor invoice but it will not reduce the COGS of the stock on the PO.
Further information
Add a Return to Vendor Against a Different Vendor
Add a Return to Vendor Against Multiple PO's
Add Restocking Fees on a Return to Vendor
Add Stock to a Return to Vendor
Handle Stock Returned on a Debtors PO
View or Edit a Return to Vendor