sold a transmission now having buyer problems

I recently sold a fella a 727 transmission that he had a shop install in a dodge camper van,

I sold the transmission for $120, and told the fella it would move forward and back when it was done if not I would give him his money back

shop calls me the next day to say they had the wrong converter (I have many 727's and mixed one up)and in the gallon of fluid they drained out they found about a half of quart of water in the bottom of the pan (was under cover whole time

gave him correct converter and never heard from him for 2 weeks

fast forward to a few days ago I seen a black dodge campervan on the side of the road not far from me with trail of fluid behind it, told the shop to change filter pan gasket and pump seal before installing
transmission


now buddy thinks I owe him $120 back because of the shops stupidity

what do I do? I don't make much money... but I don't like ripping people off...