I would request that sales persons be selected/assigned per customer and not at the time of creating an invoice.

Some of the reasons would include:
- credit sales to one customer done by several sales reps
- companies structure usually assign sales reps to customers
- difficult to compute commission and analyze outstanding amounts if sales reps are captured per invoice
- reporting becomes difficult to analyze, for example, aging report should show balances per sales person (this helps it calculating commissions).