Actually I don't think any legal case would be that complicated.
It would be about if the seller guaranteed the mare open or not. That's about it - that and if the stallion's services were gotten in some way, by the seller in an effort to not pay a costly stud fee. A DNA test might show if the pappy was one of the seller's stallions or not.
A lot of it depends on what the bill of sale actually says (does it say 'open mare' or just 'mare'). And about what due diligence the buyer did to verify that.
It's also a matter of timing. Say the foal was born on day X. The breeding date is X minus 326, give or take.
If you use the range of possible lengths of time a foal is carried, it might not be clear the mare was bred before it left the seller's farm. 'It depends'. It might be very clear. For example, the mare was at the seller's property for two years. You bought it, and in less than 300 days later, it gave birth to a foal.
Verbal agreements are considered to be forms of a contract, and can be binding in court, but there has to be some sort of verification of what was said.
And it's just as possible the new owner could be liable to the seller for stallion fees as that the seller could be liable for reproductive care, birthing care, foal vaccinations, foal board to weaning age, marketing costs to sell the weanling, further board and care if it does not sell, with the sale price of the foal going to the stallion owner. Cases have gone both ways depending on the circumstances, the state, and other issues.
You never know til all the facts are gone over by a lawyer. Livestock sale laws, are, in general, not geared toward horses as show animals, companion animals or pets, but treat them basically as livestock, like cattle.