Sold a car to a relative and it broke...

The guy I sold the car to is a nice guy, and I do not think he will hold a grudge against me what ever the outcome is. They have not asked me to come to the party here, I have decided I would. They are still not aware I intend to help to the extent we are discussing here.

I have floated the buy back idea with my partner and she is not sure that he will want us to buy it back, I explained to her that if they keep the car then I will be making it very clear about "no warranty" and there will be no gray areas.
I was thinking I would not give them an option, I would tell them what is going to happen and to what extent I would be involved, but perhaps what I could do is make 2 offers, I go 50/50 to fix the problem or I buy the car back. That way I am not pushing them into a corner and they are the ones making the decision, if they keep it and it brakes again they choose to keep the car after being offered an out, if they want me to buy it back they are on there own to find a new car.

I am going to pick the car up tonight after work, so the offer will be made then, I just need to be sure about what offer/s I am willing to make.

Part of me just wants to tell it like it is, you broke it, you pay to have it fixed.....but I realise long term that is not a good idea, going from that feeling to buying a broken car back is quite a step.

On a brighter note, I spoke to the company with the gearbox yesterday, looks like they do have the correct box and it will be expensive but it is a rebuilt unit with 12 months warranty. Waiting on the shipping quote then we will get it on the way.