Can I associate a sales person with a particular invoice?
Yes, you can associate sales person with invoice.
for that you need ledger with label SalesMan or you can create new salesman at the time of sale transaction.
1.Goto Transaction > Sale
2.Fill the transaction details.
3.at bottom left corner SalesMan field is present
4.Select Or Create new Ledger with label salesman.
5.Click on OK/Apply button.