Selling codebase

I have a SaaS solution for the logistics industry. One of my customers showed interest in buying a copy of the platform. We will brand it with their brand and install it on their server. After that, I will run my business and he can run his business independently. We decided on the price and terms. But running into some issues regarding the legal term for this arrangement.

I offered a nonexclusive, perpetual royalty-free license of the software. My client doesn't think it gives him all the rights. His lawyer proposed "acquire all right title and interest in the platform". That would leave me without rights to my site. Has anyone did anything similar and how did you word it?

In effect, I want to run my business using the software. My client can run theirs as they please, including selling or leasing it to others. Both of us will have unencumbered rights.

Any ideas?

Thanks