Studyman,
I'm not speculating because I'm not saying whether or not Abraham kept the Sabbath by not doing physical work on it.
To say that he did or did not would be speculation, unless there's a Bible verse I'm unaware of.
Whenever I hear a question like why did God do that, I'm always reminded that God's ways are not like our ways.
Whether God did or did not tell Abraham not to do physical work on the Sabbath, I believe it is not recorded in the scripture. So, I don't know If Abraham did it or not.
Jesus is the Word which became Flesh,
All things were created by Him, including the Laws and Commandments, and the Sabbath of God. So these Commandments are the Commandments of Jesus the Christ. This is the Word of God, not speculation.
Abraham did have Christ's Laws and Commandments. That is not speculation, that is God's Word.
Abraham didn't pick and choose which of Christ's Commandments he thought were worthy of his respect, and which ones he didn't. God gave Him instruction and he obeyed. And for this the Word of God says he was blessed. This is also the Word of God, no speculation.
And Christ did create a Sabbath. He sanctified it, separated it from other days and made it Holy. This is not speculation, it is the Word of God.
And Christ did include His Sabbath in the Commandments of God that David called "Righteousness". This is the Word of God, not speculation.
And Jesus said this Sabbath was created for man. And Abraham was a man. Jesus didn't say the Sabbath was created for men born after a specific time, or with a specific DNA. This is also the Word of God, not Speculation.
You and I also have God's Commandments. This is truth, not speculation.
John 14:
21 He that hath my commandments,
and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him,
and will manifest myself to him. This is the Word of God, not speculation.
Abraham obeyed the Christ and in turn, the Christ
manifested Himself to Abraham as He Promised. This is the Word of God, not speculation.
56 Your father Abraham rejoiced to see my day:
and he saw it, and was glad.
Zechariahs also honored the Christ by obeying His Commandments
and he also knew Jesus when He came. This is the Word of God, not speculation.
The Mainstream preachers of Christ's time rebelled against His Commandments and didn't follow them. This is the Word of God, not speculation.
The Pharisees didn't know Jesus when He came. This is also the Word of God, no speculation.
So you can preach that Abraham didn't have God's Sabbath that He created for man if you want.
What we do know is whatever commandments and Laws Abraham had from God, he obeyed them and the mainstream preachers had God's commandments and they didn't obey them. This is the Word of God, not speculation.
Are we to have the "Faith" of Abraham? Or the "works" of the mainstream preachers of His time?