
Same as Ells_Bells. I would immediately refund any deposit, and wouldn't expect any bills to be paid by the potential owner.
I had a pup, being sold as pet, that developed a hernia at 7 weeks (mum had chewed the cord right up to the tummy, and it bled badly.) Pup had already been chosen, and I did give the people chance to change their minds. They had already met him several times, and still wanted him. Instead of him leaving at 8 weeks, I had the hernia operated on, and he stayed with me for recuperation. I paid for the operation, and it meant that I actually got a couple of hundred pounds for the pup.
But his care was my responsibility, and I preferred to see to his aftercare myself, and the people he went to had already been vetted, were a smashing family and I knew he would be much loved. The right home was more important than money.