How to do I reduce the balance on an invoice for a donation?
If you would like to reduce your customer's invoice and book the discount to a donation, I recommend creating a new item called Donation. When creating the item, you will want to choose the bookkeeping account that you use to track donations. You can enter 0.00 as the amount and any unit you want. Leaving the amount as 0.00 will let you update the amount per invoice.
Once you have created the item, add the item to the invoice and enter in the amount as a negative. So if you want to donate $250.00, enter in -250.00. This will reduce the invoice amount by 250.00 and record the donation in the bookkeeping account you specified.