When you submit the work, there will be a place for the amount you're requesting from escrow. Normally, you'd just leave it at the full amount for the milestone, but in this case, change it to $100. The remainder will be refunded to the client when the contract is closed.
The change in the amount won't affect your JSS.
Contract names can't be changed. You can discuss this with the client to hire you on the new contract. Having a new project will enable you to precisely define the job scope and requirements since those are not the same as for the previous task.