The Harvest.

A screenwriter is sent to Mexico to develop a story that can be used to wrap up a movie presently in production. Given a very short time allotment, the writer immerses himself in the seamier side of a Mexican beach resort, but soon finds himself involved in a 'body organ for profit' operation. Drugged, he has one of his own kidneys removed, and then, with a woman he isn't sure he can trust, he sets out to expose the ring...