The best thing you can do is strip the whole scene bare( remove HDRI/Lights), apply basic corona material on everything you have. Set render settings to default.
You give them the scene but its worthless. And you did hold up to your agreement.
True. I would also want to add that if you want to maintain good relationship with your client, make it clear that you cannot give them assets that weren't done exclusively for him and also that you cannot give nor resell assets that you bought and used in the job you did for him.
I also work for a client that requests all source files. But at the very beginning we agreed that I will not include anything but things I did and only those I did for them! Also I wouldn't agree even to that if I had any suspicion they would be able to use it in any way.. They just want to have it..
If your task is to do some stills or animation, then your output/product are those stills or animation. So, them requesting your working files and assets is close to buying a car but also asking for all the machinery and plans on how to build one.
It's actually funny that you had such a thing in the contract and didn't notice that..