Preventing someone from entering a value in a field at an early sales phase while having it required

Hello.

I want to be able to prevent users from filling out a field at our earliest sales phase, but allow them to enter data once the deal has reached a later sales phase.

So for example, I dont want people to put in a deal $ value at the first sales stage because that is too early to know how big the deal is, but I do want them (and it's a required field at this point) to be able to enter a value when it gets to a later field.

Thanks.
Glen