If its not warranty work there is no obligation to keep the Cable for a crrective measure to be put in place.
If a quote given is belonging to an out of warranty product and the quote given is to supply a free service, it does not have any gravitas if the owner requests their Cable to be returned.
Holding on to the Cable without giving a full explanation, creates a situation where theft of the Cable can be suspected as an intention.
A Reputation of the Sales Service 'Revelation Audio Labs' which is dependent on making sales, is on the verge of being trounced, if a proper dialogue does not develop to reunite the Cable and its owner.
Revelation Audio Labs should take heed, as the Gon Mob with their Stick Torches and Pitch Forks in hand are very adept at driving the unwanted types with strange behaviours out of the Village.