Actus incaeptus cujus perfectio pendet, ex voluntate partium, revocari potest; si autem pendet ex voluntate tertia personae, vel ex contingenti, revocari non potest. An act already begun, the completion of which depends upon the will of the parties, may be recalled; but if it depend on the consent of a third person, or of a contingency, it cannot be recalled. Bacon's Max. Reg. 20.