How does a business split up rights to code?

I’m working on an app, and have been using ideas from a client on how to make an app that will be valuable in their field.

We decided that we want to create a business and split the revenue 50/50, but they don’t want me to be able to reuse my code for my other projects, even if my project apps are not targeting the same market.

I feel like this is unfair since I wrote the code before we even created any kind of business together.

To be fair I did work with him and built most of this app with him telling me about his daily work.

I simply want to be able to reuse the code for other non-competing projects.

Any recommendations are welcome!