Return it Sunday, he said, and now we surf together

My surfboard fin broke at El Tunco on a Saturday and the nearest shop was closed. A stranger on the beach pulled a spare out of his bag, helped me screw it in, and said, return it Sunday. On Sunday we surfed together and the fin ended up staying with me. He says he is still waiting for it. I am still waiting for him to stop being kind. We have not resolved either problem. We surf on Sundays. The fin has started to feel like a small, shared animal.