
I had this happen 7 years ago, in exactly the same situation )holding pup to 16 weeks, though didn't realise at the start it would be that long).
Mine actually broke her leg and I then had to show the new owners that she was sound including a Vets opinion of course until they deigned to pay for her. I had told them payment was expected when she was 8 weeks old, but never made a big deal about them not paying then and two weeks later she broke her leg.
What it taught me was to e sure to have the pup fully paid for and insured in the new owners name, as in my case I ended up spending the price of the pup on her treatment.
I would get a Vets statement saying what the diagnosis and the prognosis is, and also that the pup is cured if you keep her until then.