The Submit Revision button on the Revise Order screen is greyed out until the Balance due in the Order Summary is $0.00. WithWine won't save a revision while money is still owed to or by the customer.
If Balance due (refund) shows a minus amount
The new order costs less than the customer paid, but you haven't said how to give the money back. For example, Balance due (refund) -$23.80.
To fix it, do one of these:
Under Existing Payments, change the payment from Retain to Refund, and check the Refund Amount. This is a partial refund. See "How to give a partial refund".
If you don't want to refund the customer, take the extra off another way. For example, add a product of the same value.
If Balance due shows an amount owing
The new order costs more than the customer paid. To fix it, do one of these:
Take a payment under Additional Payments, by Card or Cash. The button becomes Pay & Submit Revision. See "How to add products to a paid order".
Add an Additional Discount for the amount owing, if you're not charging the customer. The balance goes to $0.00 and the button becomes Submit Revision.
If the order has more than one payment
Check every payment under Existing Payments. Each payment has its own Retain or Refund setting. When an order has more than one payment, WithWine leaves them all on Retain, so Submit Revision stays greyed out until you choose which payment to refund. The refund amounts together need to match what you're refunding.