I want to be able Update an existing Transaction and associate it a affiliate.

My use I have people who when they refer someone to my 5 day challenge if they purchase at the end then they will be the affiliate they will get the affiliate Commission But the purchase isn't done through a funnel it's done through a phone call which is very different and there's no way I see to associate that lead over to that sail so I want to create a custom association field and just associate that with my affiliate contact and through a workflow anytime that a purchase is made it's going to check that associated contact fields and then assign that purchase of that type of product to the affiliate that is in the associated contact field via a workflow.