Computers exist. You can track any financial transaction done via computers. The FBI is quite adept at that, they got *really* good by tracking global terrorism funding post 9/11.
Not sure why computers existing matters. A transaction was made. The FBI knows this. They know Ippei physically requested the transaction. They just don't have access to security footage of Ohtani 24 hours a day to verify whether Ohtani ordered the transaction or not.
Imagine Jordan and Ohtani gambling over the coin toss game back in the day. "Black Sam" Carlisi and the Chicago Outfit forced to go to war with the Yakuza.