i was going to suggest purchasing a new high flow cat.. and on-sell your highflow and dump to someone else.. would be able to re-coup some of the money you spent on the high flow.. and someone would be interested in highflow..
would have obviously lost money, but would have been able to do the work yourself at home.. and prob would have cost ess then what you paid for his services..
You should have made a verbal agreement for use of his time and equipment.. I'm sure there would be legalities of having you in the workshop. Hence not writing down itemised list.. In my workshop, if someone has an accident, etc.. we are in the shit
Also saying that is cash price is bullshit to be honest.. I would have told him i'd need to get the cash and not return.. You can pay with whatever option is available..