Got a client who wants to maintain a adding API URLs to my code I developed. Wants me to train him on it so that he doesn’t have to wait on me. How do I protect my code from being used without my authorization?

Got a client who wants to maintain a adding API URLs to my code I developed. Wants me to train him on it so that he doesn't have to wait on me. How do I protect my code from being used without my authorization?

I developed some code for a customer and every so often they need a new URL added to the list that the code fetches data from. The client doesn't want to wait on me, not even three days to take care of an issue with it so he is asking me to show him how to maintain the code himself. Says he will pay me.

What's your guys thoughts on client relationships like this? How can I protect my code from being used without my authorization? What I coded could potentially be sold over and over again as a solution. So I'm hesitant to let him have access to it.