If payment has not been made then warranty does not exist, for warranty to exist the original contract has to be completed and that includes the clients half of the original arrangement/obligation. Perhaps come to a compromise with him/the company and have your payment ready in cash or some other guaranteed from that he can accept at your doorstep before he comes in to see what's up. They get paid, you have the repair person on site, seems like a mutually-agreeable compromise. As a service tech I encounter call backs that are sometimes warranty and sometimes something new. If fault lies in my materials or workmanship I am eager to correct it, if something else has happened then I review that in detail. A recent one was a neighbor two doors over whose garburetor had jammed, I had installed it about a year ago. He was wondering if the unit was defective but I showed him the American dime stuck in between the spinning grinder 'platter' and the outside of the grind chamber. Darned thing took a while to get out too.
I finally got an EMT from a deadbeat realtor today. The original service call was last March! He was always nice on the phone but I could tell he was just trying to get his realty client to pay rather than just ante up like he should and then either collect or not from the other guy, I could really care less how that transpired. He was the one who had established the contract with me, not his client who needed a weird heating system checked over. An in-person visit to his realty office and handing over the invoice to the other staff there somehow managed to light the fire under his butt that didn't exist previously