Reminds me of a post on this forum a few months ago where someone who was a mechanic had some customer ring and complain their engine had siezed or something from a service they did a long time ago, assumed said mechanic had forgotten the oil on that service, and wanted them to pay for the engine work. Same general principle anyway. Noone wants it to be their fault.
As you can see I'm very clear on the details
Refer them to the receipt you gave them, specifically the part where it says "as inspected and accepted".