which party is liable when a subcontractor fails to invoice a customer directly when a general contractor explicitly refers a customer to them?

a homeowner in a condominium association had a water leak unknown to them. i contacted the homeowner and offered the services of a plumber i frequently referred customers to. i contacted the plumber and instructed them to bill the customer directly. i contacted the technician at the time of service to remind him to invoice the customer. when i received the invoice in error i contacted the plumber and told him to bill the customer. two years later i receive a collection notice and now it is on my credit report. i have contacted the customer (a couple in their 90's) and they asked for a bill at the time of service. the plumber refuses to respond, the collection agency refuses, the credit bureaus refuse to respond to my explanation of the circumstances. how can this be deleted from my credit file? can i be sent to collection for assisting a subcontractor and a customer with a referral? i made it clear to the plumber they were to bill the customer and after two years i get a collection