Genesis 25:34

And Jakob gave to Esau bread and pottage fo lentiles. And he ate and drank, and arose and went. And Esau scorned the birthright, and the portion of the world that commeth.

Eisov scorned the birthright. He saw that his father had become poor in his old age and that there was little for him to inherit.

SO ESAU DESPISED HIS BIRTHRIGHT. Who so despiseth the word shall suffer thereby. since, as is clearly indicated in Scripture later on, Esau regretted his action and complained that he [Jacob] took my birthright. (Further, 27:36.) Scripture therefore justifies what befell Esau later on by saying here, So Esau ‘despised’ the birthright, and he who despises the word shall suffer thereby. But, indeed, Scripture has already explained the reason that Esau consented to the sale. This was because he was in mortal danger from his hunting animals, and it was likely that he would die while his father was alive, and the birthright carried with it no distinction except after the passing of the father. So of what benefit was the birthright to him? This then is what Scripture says: And he did eat and drink, and he rose and went, and he despised, for, after having eaten and drunk, he returned to his hunt in the field which was the cause of the despising of the birthright. For there is no desire in fools except to eat and drink and to fulfill their momentary desire, not giving a care for tomorrow.
Now Rabbi Abraham ibn Ezra has erred here exceedingly by saying that Esau despised the birthright because he saw his father destitute of wealth. Now, [continues Ibn Ezra,] many wonder about Isaac’s poverty since Abraham left him with great wealth. But have they never seen a person who was wealthy in his younger years and became poor in his old age? An indication that Isaac was indeed poor is the fact that Isaac loved Esau because of his venison. Furthermore, had there been abundant food in his father’s house and he [Esau] “the honorable one in his sight,” he would not have sold his birthright for pottage. Also, if his father ate savory meat every day, what reason was there for him to say to Esau, “Bring me some venison?” Why did Jacob not have costly garments as Esau had?, Verse 15. Why did his mother not give Jacob some silver and gold for his journey when he fled to Haran so that he had to say, And He will give me bread to eat, and a garment to put on?, 28:20. Why did she not send him some money — since she loved him — so that he was required to tend Laban’s flock? The verse which states, Thus the man grew [in wealth], must refer to the period before he became old. Now ignorant people think that wealth is a great distinction for the righteous. Let Elijah prove the contrary. These ignorant people further …

ויבז עשו, seeing that eventually Esau regretted having sold his birthright, as we know from 27,36 the Torah goes on record here that he had despised the birthright and had no right to go back on the bargain he had made in full knowledge of what it entailed.

ויבז, the word is equivalent to Numbers 15,31 ויבזה from את דבר ה' בזה, the construction being parallel to ויקן as a form of the past tense קנה, “he bought, he acquired” (Genesis 33,19)

ויעקב נתן לעשו, he had used the lentils or the dish containing them as the קנין חליפין, a token used to serve as symbol of the transfer of ownership of the object constituting the one being sold. Compare Ruth 4,7 where a shoe is mentioned as serving as such a token.

ויבז עשו את הבכורה, even after the transaction and the meal the birthright was not worth the price he had received in exchange for it. It is clear therefore that the seller had not been taken advantage of seeing that transaction involved so little of value.

ויעקב נתן לעשו, as the price for the birthright. He is not reported as having given Esau wine, as the wine he drank was that belonging to the household of his father. However, fresh bread, which Yaakov had baked for himself, he did give him together with the dish of lentils. My father, of blessed memory, wrote that Yaakov positively compensated Esau with money, and that the dish of lentils and the bread he gave him and which they both ate together was only to seal the bargain between them, a custom to which we have already referred. [It is difficult to know whether our author adopted this interpretation, or whether he did not consider it as the plain meaning of the text as written. Personally, seeing that the author generally distinguishes between a past tense constructed with a letter ו when appearing as a prefix to a future mode, and the ordinary past tense, the word נתן instead of ויתן should have made him adopt his father’s interpretation as the plain meaning. Ed.]

וילך, he returned to the field to continue hunting.

ויבז, even after he had eaten and drunk he did not feel that Yaakov had tricked him and taken advantage of him when he had felt hungry.

ויבז עשו את הבכורה, “Esau had already displayed disdain for the asset known as birthright.” This is why Yaakov bought it from him. We find that Rabbi Yehudah Hachassid, referring to this verse, claims that if one finds a confirmed sinner in the possession of holy texts such as a Torah scroll, it is permissible to use subterfuge to get him to sell it.