IANAL, but they either have to honor the verbal contract (pour an additional cup AND pay $20) OR the contract is void and therefore they can be sued for assault. Of course, the "can" in "if I can" may be construed as "being able to", but that's up to the jury I guess.

Pouring more water isn't required, only the permission to pour more water is required. And that was granted in this scenario.

As the old saying can go, "It depends on what the meaning of 'can' can be."