Back to the original question, you want a user name on the sales slip.
Normally I setup AR Salespersons with the same code as the ePOS user ID so that when a sale is made in ePOS the salesperson is set to the ePOS user by default, this is the way ePOS works.
So if your ePOS user ID = AR salesperson code then you can print the salesperson code (which is on the receipt slip by default) on the receipt slip, that should solve your problem.