How do we plan out getting paid for roll out delays? Draft a structured plan in the proposal? Y$ (>X$) for the first 200 users in the first month paid upfront? And scale down Y to X as we scale up the number of users?

More background: Our first enterprise customer has verbally agreed to  set up a commercial agreement. We negotiated a price per user X$ /user /year . But I need help with the roll out plans. Our champion in the enterprise says that they can't get to rolling out a 1000 users till a few months and to them there is no point paying for users who are not signed on.

An idea, do you have to customize their account in any way or will it be pure SaaS?  If you have to customize, perhaps a setup fee so that you can get some small amount up front, then X months free, then annually up front for the 1000?