It hit me today that Yaakov paid for the bechora in cash, not in soup.
"And Eisav sold the bechora to Yaakov" and then "And Yaakov gave to Eisav bread and lentil soil."
I think the simple pshat is that the soup was unrelated to the transaction. It seems many meforshim agree (though Ramban and Ohr Hachaim do not).
There is a beautiful pshat based around that, but it is based on the premise that he indeed sold it with the soup.
A group of bochurim in Europe got lost in a forest, and after wandering for days, they chanced upon the hut of an old Jewish lady. Famished, they begged the woman for food, but sensing an opportunity, she said that she'll only give them food if they promise her a tremendous sum of money. Having no choice, they reluctantly agreed, and she gave them some food.
After regaining some strength, the woman demanded that they pay up, but they objected. It was Pikuach nefesh, they said, and they should not have to pay the exorbitant sum. The woman took them to the godal at the time (IIRC, it was Rav Yitzchok Elechonon Spector), insisting that they pay her the money. The Godal ruled that they were not obligated to pay.
"Why does it say that Yaakov gave Eisav lechem u'nizid adashim? Eisav didn't ask for bread, only for soup? The reason is because had he just given him soup, Eisav could've gotten out of the agreement by asserting that he was dying, and that Yaakov was obligated to give him the soup to save him. Thus, Yaakov first gave him bread, and only after Eisav ate did he complete the transaction with the soup."