Proper consideration requires a promise before an act. A voluntary act followed by a promise isn’t valid consideration.
Harrington v. Taylor, 225 N.C. 690 (1945)
The plaintiff in this case sought to recover of the defendant upon a promise made by him under the following peculiar circumstances:
The defendant had assaulted his wife, who took refuge in plaintiff's house. The next day the defendant gained access to the house and began another assault upon his wife. The defendant's wife knocked him down with an axe, and was on the point of cutting his head open or decapitating him while he was laying on the floor, and the plaintiff intervened, caught the axe as it was descending, and the blow intended for defendant fell upon her hand, mutilating it badly, but saving defendant's life.
Subsequently, defendant orally promised to pay the plaintiff her damages; but, after paying a small sum, failed to pay anything more. So, substantially, states the complaint.
Whether there was a consideration recognized by our law as sufficient to support the promise.
Consideration requires a promise made in exchange for something prior to the act. That was not the case here. (implied)
However much the defendant should be impelled by common gratitude to alleviate the plaintiff's misfortune, a humanitarian act of this kind, voluntarily performed, is not such consideration as would entitle her to recover at law.