I am trying to implement a sales commission rule.

Sales from the website (using appointments to sell services) & sales from the POS.

If the employee reaches 4000 (CURRENCY) in sales, they will gain a 3% commission from the excess sales
Example: Employee managed 6000 USD sales. It would be 6000-4000 = 2000 x 0.03 = 60 USD.

My biggest problem is attributing the sales specifically to employees. Imagine if I sold two services, one should be attributed to one employee and the second to another but the way sales work you can place only one salesperson. 

I might be overlooking something so please let me know