1396. An offer to contract made to a determinate person constitutes a promise to enter into the proposed contract from the moment that the offeree clearly indicates to the offeror that he intends to consider the offer and reply to it within a reasonable time or within the time stated therein.
A mere promise is not equivalent to the proposed contract; however, where the beneficiary of the promise accepts the promise or takes up his option, both he and the promisor are bound to enter into the contract, unless the beneficiary decides to enter into the contract immediately.