That would depend on contract, but, assuming you have a decently sane contract, you'd have to hand over the script, but not the software itself. However, it now sounds like you've landed your first paying customer for a license for your software.

I was in a similar position in my first job. We got an extremely detailed functional description of what the software was supposed to do. It was so detailed, I had a couple of scripts and macros that created most of the code for me. I was always the first to be done with my work. I wouldn't have minded sharing those macros, in fact. Those macros were specific to that particular project, though.

I was eventually fired from that job (I suspect my boss didn't like seeing me lean back and watch my scripts and macros run), and this never came up. I think the real value here is the skill and desire to automate these things, not the automation scripts themselves.