I have a dash rattle, that took a year for me to figure out the source. On mine its under the HVAC controls, the plastic above the cell phone sliding door. For the longest time it sounded like it was buried in the dash.
I would let him keep it. You would end up with an unplated uninsured vehicle, and that is a hassle to try and sell. Sounds like you are a good guy for even considering taking it back, but his mistake shouldn't become your migraine.
Hello All. Just brought home a 2022 Lariat with a 5.0, and loving it. Last truck I owned was a 1988 F150, but they are worlds apart, the current generation are really impressive. I live where it snows, and needed something when the Mustang is parked for a few months.