Well I have to say, I did enter into a sale like that, bought a bred pony years ago for the kids and the owners wanted the foal. We did agree before hand, at what age the foal was to be weaned, and they did pay for board for the foal once it was born. It worked out ok, I got to experience my first foal and they got the baby they wanted. But with this one, I don't know if I would cause, Do I really want a foal around at this time if it's not colored? Can I afford to keep an extra horse that I hadn't really planned for in my long term plans. and it is very very hard to let the foal go when it comes to weaning time. And your right lots can go wrong.
Last horse I purchased was also bred and I was given the option of keeping the foal or giving it back. And yes there was a price difference on the mare. I chose to keep the foal and pay the higher price for the mare because I didn't want to go through that again. (and heck how hard could it be to sell an appaloosa foal?) Well as usual I made the wrong choice, I paid the higher price for the mare only to find out later on that the guy never had the mare vet checked when she was first thought to be pregnant. She was, with twins, Lost them both at 9 months in. So it can go either way. When I told the previous owner about the loss of the foals, (I was hoping for the refund of the price difference, maybe, wasn't holding my breath) he did offer a free breeding back to the stallion, and it was tempting, but I really didn't want the foal anyway would have sold it so I passed.