No wrong at all - you and he agreed you would do the work on his car for £250 (as he didn't help) the extra gearbox was worth circa £250 so as he didn't pay you actual cash I'd say it perfectly fair for you to sell the box and keep the money or use it yourself - so yeah saying call it quits is perfectly fair in fact leaves you £20 down if anything! He wouldn't have had a car to insure if you hadn't built it for him! You be a 'mate' and people take the piss and take advantage, some mate eh

You are not in the wrong

Z