Exactly- you need to pay the mare off in full and draw up a contract stating that the foal is yours and agree upon a reasonable (to both parties) amount of remuneration for their time in foaling her out, keeping her and the foal, and also state that any vet, farrier and feed bills for the mare and foal are your responsibility. BUT GET IT IN WRITING! AND DO IT NOW! While sales most often go exactly as planned, its always best for both parties to protect themselves with a written contract. It should include terms about what will happen if there is a problem at foaling and the mare, foal, or both is lost. (or if an accident or illness at any other time results in the loss of one or both). It should include what is to happen if you do not wish to keep the foal after she foals out-if it isn't what you expected and you decide to sell, does the seller want first right? But most of all, if you're determined to have the foal in the price of the mare- GET IT IN WRITING and pay the mare off in full before the foaling happens so the seller cannot change their mind and decide to charge extra for the foal (not just foaling and board) -or just flat out refuse to sell it to you.If the delivery is post poned till after foaling then payment should be made in foal immediately and a written agreement that any costs, vets etc. are the buyers responsibility and some remuneration for "mare stare"