the Law hadn't been given at that point
Now we are getting to the purpose of this digression. Allowing different things at different times and not allowing them at other times DOES NOT mean that someone (namely me) throws out the law or disregards Moses. The patriarchs clearly had some aspects of the law since we know they sacrificed, and we already touched on principles of the Levirate marriage in Judah's day. Abraham had the gospel preached to him despite not having the benefit of what Matthew Mark Luke and John wrote.
Gal 3:8 And the Scripture, foreseeing that G-d would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, “In you shall all the nations be blessed.”
We simply do not know in what way this gospel information was communicated to Abraham. Neither do we know to exactly what extent Moses merely documented God's law that already existed (including at least some sacrifice, some aspects of the Levirate marriage) and what was new ("when you come into the land").
For instance, this part of the law related to the events of the Exodus:
Exo 13:11 “
When the L-RD brings you into the land of the Canaanites, as he swore to you and your fathers, and shall give it to you,
Exo 13:12 you shall set apart to the L-RD all that first opens the womb. All the firstborn of your animals that are males shall be the L-RD's.
Exo 13:13 Every firstborn of a donkey you shall redeem with a lamb, or if you will not redeem it you shall break its neck. Every firstborn of man among your sons you shall redeem.
Exo 13:14 And when in time to come your son asks you,
‘What does this mean?’ you shall say to him, ‘By a strong hand the L-RD brought us out of Egypt, from the house of slavery.
And when did Moses write the law? When Abraham's children stopped being a family (Genesis) and became a nation (Exodus). And that is what I meant above when I said that that the Law of Moses was a law for a nation.