Uncertainty that prevents the formation of a binding contract — including a condition requiring payment of a 'reasonable sum' — does not necessarily prevent a proprietary estoppel from arising. Where promisors have themselves subsequently determined the content of an uncertain condition (here, by executing wills specifying the amount), the court may use that determination to frame the conditions of equitable relief. A constructive trust vesting beneficial ownership is the appropriate remedy where the detriment suffered is the loss of the property the claimant worked to improve in order to gain the proprietary interest.
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