You owe her nothing. Without a written contract stating that you would give her half of the sale money, her argument would not hold up in a court of law. Even if she happened to have slipped it into a conversation, and you just forgot about it, there's still nothing that she could do. Everything HAS to be written down, or it's all gibberish.
I agree with another poster- make up a contract for future occasions. My cousin decided to take a free horse from a woman, to have it retrained by a friend, then resell. She made the woman sign a contract before they even touched the horse, stating that the horse was legally my cousin's after that point, and that every decision made about that horse was hers to make, and hers alone. It started as just a precaution, but the horse was spoiled, and they had to discipline it before they got off the place. It flipped out, reared up and fell over on itself, but thankfully didn't get hurt. It acted very respectful afterwards, and listened to everything they asked of it. The woman freaked out, called the police and told them they were "abusing her horse." Once the police saw the contract, and heard the story from my cousin (who is a vet, and does NOT abuse animals), they told the woman that she no longer had control over the horse. It was a quick end to what could have been a long legal battle. My cousin probably would have won the legal battle, but it would have been tedious, time consuming, and costly.
It's sad that we live in a world where we can't trust anyone anymore, but it's best to be protected.