In this paper, we develop a contingent claim analysis on shareholders' right to sell unconditionally their shares at the acquisition bid price during a takeover bid procedure. Compared with a situation without any guarantee, this regulation brings about wealth transfer towards outside shareholders. Why, in an apparently irrational way, do outside shareholders, who may benefit from a price guarantee, not systematically sell their shares? That question emphasizes the outside shareholders' behavior. Using a real option valuation model to evaluate the price guarantee opportunity, we show that an equal treatment rule between controlling and outside shareholders may lead outside shareholders to sell their shares.