I created a refund for an Payment Txn and I added the Total Amount Paid field to the Opportunity Layout so it would reflect the actual totals, but it is still blank, even after an edit/save. How does it get updated?
If the Total Amount Paid field owing is added after all txns are created (payment, refund, etc), it needs at least one txn to go through the matching process to update that field on the opportunity.
Change any related txn to Matching Start, and when it goes to Matching Complete the field will update. Only one txn is required to do this step, not all related txns.