A legal obligation is not the same as a moral obligation. There is a formality in the law, especially the law of contracts, that sets it apart from the dictates of justice.
The patriarchal narratives in Genesis derive much of their dramatic force—not to mention instructive power—from this tension between legal and moral standards. That is one reason for their timeless appeal. The chasm between legal and moral responsibility has never been fully bridged. Legal forms, in the hands of the unscrupulous, can be an instrument of injustice, in ancient times as today.
In some Biblical contracts a solemn oath creates an irrevocable commitment, even though given under a misapprehension or obtained by deceit. Take Jacob, for example, the younger twin. By ancient custom his elder brother, Esau, was entitled to a double portion of inheritance from their father Isaac. Jacob attempts to correct this misfortune by a series of legal maneuvers.
First, when Esau is famished and requests some of Jacob’s lentil stew, Jacob demands his elder brother’s inheritance in exchange (Genesis 25:29–32). Technically this may be a valid legal contract, but readers easily perceive the injustice of such a lopsided deal. Jacob has acted unjustly by exploiting his starving brother’s weakness. Formally, it is a valid legal bargain, but morally, Jacob has taken unfair advantage of his starving brother.