When, according to usage or the nature of the contract, or its own terms, an offer made to a particular offeree can be accepted only by rendering a completed performance, the offeror cannot revoke the offer, once the offeree has begun to perform, for the reasonable time necessary to complete the performance. The offeree, however, is not bound to complete the performance he has begun.
The offeror's duty of performance is conditional on completion or tender of the requested performance.