Change Price on a Billed Sales Order and Paid Invoice

To reflect a price of an Item decreasing on a Sales Order that has been Billed and an Invoice that has been paid the following can be done:

1. Edit the Sales Order and change the price in the Amount column. You will receive the notice: Items on this line have been fulfilled and billed. Are you sure you want to modify it?
2. Click OK. 
3. Click Save.
4. Edit the connected Invoice and change the price in the Amount column.
5. Click Save. You will receive the notice: This invoice has payments applied to it. The payments will be applied differently. Are you sure you want to edit it?
6. Click OK.

The payment amount applied to the Invoice will decrease to the new total of the Invoice and the remaining payment amount will remain as a credit on the Customers account. This amount can then be refunded or applied to the next Invoice.