“Sign up and pay at least part of it now and we’ll prioritize the feature”.
I’ve seen enough instances of work being done for a specific customer that doesn’t then result in the customer signing up (or - once they see they can postpone signing the big contract by continuing to ask for “just one more crucial feature”, they continue to do so) to ever fall for this again.
Why do that if your competitor already has it? I'd just go talk to the competitor instead. If you aren't able to ballpark when the feature will be done, why should I trust you will once I pay part of the price?
Because you have other benefits, so we'd really like to switch over to you, but we can't unless you support this dealbreaker feature that your competitor we're currently using has.
I’ve seen enough instances of work being done for a specific customer that doesn’t then result in the customer signing up (or - once they see they can postpone signing the big contract by continuing to ask for “just one more crucial feature”, they continue to do so) to ever fall for this again.