As part of the vendor payment unsettlement process, D365FO users will need to manually go through each line and unsettle / reverse each transaction. This is after the vendor refund payment has been received in the bank account. There needs to be a more efficient method of completing this task as there will be journals with 100s of lines that will need to be reversed per journal. 

Currently Microsoft have proposed the following (as part of their Microsoft Doc here);

1. Navigate to the Vendor Master Record and select Transactions in the top pane.

2. Navigate to your Vendor Payment Transaction, select Settle Transactions and Undo Settlement. 

3. Once this is complete, complete a manual reversal of the Vendor Payment Journal. 

4. Settle the Reversal of the Reversed Payment Journal and the Old Payment so this doesn't populate in your next pay run. 

5. Once these steps are complete, you will be able to run a new payment against the original vendor invoice.