One method to resolve this error:
- Look up the donation form and donation campaign for the existing transaction record
- Identify the original merchant account for the transaction in a Transaction Details Report
- Alter the Merchant Account of the same Donation Campaign back to the original Merchant Account
- Attempt the merge again
- Refund the transaction "offline"
- Merge the records
- Set up the same gift on the surviving contact record as "offline" for your reporting purposes