A question of "ethics" (update)
That and the fact that you more than likely just picked up another customer for life. Who hopefully will also tell their friends what a stand up guy you are, sending more business your way. Which is exactly the way business should be done.
Sending the man a receipt for him paying the invoices is disclosing that you have been paid and for what.
When two people send payment for the same invoice then you need to call everyone cause someone has messsed up and it probably was not the man that did the work.
My biggest ethical problem with this is how it went down for paden cash by the fact that his invoice was passed on to someone else.
I have a policy that whoever is responsible for paying me is the one that orders the work and gets the invoice.
Don't like the idea of middle men cause they don't pay and have no say in what really needs to be done. All they usually do is interfere with costs and requirements cause they are counting on the override to pad their pockets.