I don't get it... what the joke means??

He gave her a ring when they met in town and said to go to his shop and show it to them and she could have anything she wanted(if I remember it all correct) and he told her at that time it would be hard to sell the ring. So when he was surprised she didn't actually go to the shop and she said she'd thought to sell the ring it was a joke about the last time they met.