Enter as a Payable Charge to the supplier account as a negative number. If the credit is for $50, you will have a payable charge of -$50.00 (negative $50.00). Choose EX and the expense account to which the $50 should be credited. This will also appear as a negative number. If the discount is a regular occurrence (I work with some clients who receive a discount on feed if paid within 7 days), then you may consider setting up an account called "Less: Discounts" and record the discounts to this account. Feed Expense would be one account and directly below it would be the account called Less Discounts.
