I think you have a meeting to discuss the request and if you’re happy to accept their suggestion instead that’s what gets agreed and your contract gets changed accordingly. It also depends if you’ve submitted a statutory request or if it’s just informal. I’m no expert but just been through it myself so did a lot of research
I don't think you can negotiate it that way. But in your initial application and through the process you can discuss alternatives and say if your first choice isn't approved, here's the second choice etc.