Jacob says, ‘Well, make an oath to that effect. State it clearly.
This is the trouble with sin – it is too costly. What we get in return is too small and soon gone. It is like paying with blood by the bucketful. Esau enjoys his stew but how soon it is swallowed and how soon the pleasure of it is over. Then what remains but to pay up the agreed price. When the pleasure of sin is past and the prospect of judgment fills the horizon, the bargains we have struck in this world don’t seem so marvellous. Jacob is canny enough to anticipate this change of perspective and locks Esau down with an oath.