To view the original details of a transaction downloaded from your bank in Banktivity for Mac, double-click the transaction as if you were editing it and click on the icon that looks like a lowercase i next to the payee field. This is the case for transactions which you have edited and saved, and even works for merged transactions.

Note that merged transactions are created when you resolve duplicates manually. While Banktivity does a great job at matching transactions and eliminating duplicates automatically, occasionally a duplicate transaction gets through. When this happens you can merge them by simply dragging and dropping one of the duplicate transactions over the other, as long as they are from two different sources (e.g. a manually entered one and a downloaded one).