Q: When do business transactions stop on Friday? First azan or second. Would transaction done after first azan go through?

A: Better to stop at first azan.  Completing a quick transaction that would not delay you for the khutba would be permissible.

A deal done as one is closing operations and going for the jummah salah after the first azan would be permissible and the transaction would not be viewed as forbidden.

To wait purposefully for the second azan to stop all transactions is disliked.

Allah Certainly Knows Best.