[chesspain]

After reading nearly all of the replies so far, I think that the fairest solution is to allow the interrupted bettor to bet what he wants, and then give the premature caller the option to either:

1) Call the bet.
2) Fold his hand while calling the amount of money already placed forward by the bettor.

This would seem to protect the bettor to some degree (e.g. he gets some money from the caller risk-free), while punishing the bettor if he gets overly greedy (e.g. he gets no more than the minimum he has put out even if the caller might have called a reasonably sized bet rather than be faced with an exorbitant bet).