According to Jeff Selingo’s book Who Gets In and Why, it depends on the school.
Colleges don’t like to admit that they add extra merit aid to attract an admitted student who hasn’t confirmed they will attend — but some do.
Other schools might want to reserve their biggest awards for students they feel confident will accept and be great ambassadors and cheerleaders for the school.
I think it depends on how badly the school wants the student, how much money they have available to offer, and their calculation of how much money would make the difference in the student’s decision-making. And not every school takes the same approach.