[checkoutpaypal] discount on price per code option?

I am looking for a solution to allow a discount on the price for the total cart amount.

-If a sales person refers a customer to the site, I would like the customer to enter a predetermined discount code to receive his/her discount.
-This code will not only give them a specific percentage off, but will also allow me to track each sale to its specific sales person.
-Each sales person will be assigned a discount code to give to their customers.

This can be very useful for many users. Other possible uses can be to track sales without giving discounts. So, when I receive the receipt from Paypal it will have an itemized section displaying the code and any discount that may or may not apply. Any ideas will be greatly appreciated.