What software can I use to make sales contracts?

I'm a software developer and have been working on https://blitapp.com for the past year. I've had prospective customers ask about certain features that I have not yet implemented in the product. Ideally, I could send them a sales contract where we could strike a deal. After they sign the contract, and if I implement their feature requests on time, then they are required to pay and we can close the deal. I currently use Braintree payments and am not aware of how to do anything like this. Has anyone made sales contracts like this? What software/services do you use for deals like this?