Laws of the Birthright

We read in Nachum Sarna's article that there were certain laws about giving the birthright in the time when יצחק and יעקב were alive. The definition that Nachum Sarna gives for "birthright" is "the special status accorded the first born in the biblical world." Apparently, it was not a strange thing to do to give the birthright to a specific son regardless of birth order. The laws of the time did say that if he has two wives, one he loves and one he doesn't, he cannot give the double inheritance to the younger son even if he is the son of the loved wife.

There are three ways according to Nachum Sarna that the possession of the birthright can be passed on from one son to another. It can either be predestined (given to a particular child before either of the children are even born), traded between the two as יעקב and עשו did, or switched because of a major wrongdoing on the part of the elder.

What doesn't make sense is that if the way יעקב traded with עשו for the בכורה is allowed in this, why would Nachum Sarna say that this was the reason for all of his hardships later in life?

As we mentioned earlier last quarter, אברהם and his descendants were to be known for their צדק ומשפט. While it may have been legally acceptable for יעקב to obtain the בכורה the way he did, it might not have been done in a moral/ethical way. There might have been a much better way to do it, and that could be a reason that he had hardships after (even if the hardships weren't all because of this event).