We must recognize that Abraham and Sarah were observant of at least these two laws umder under the Code of Hammurabi, a Babylonian codification of law, the Nuzi Tablets and old Assyrian marriage contracts, stating that
a woman could give her maid to her husband as a second wife, to bear children for her, to keep her husband from divorcing her, if they had been childless for 10 years" which was a common practice of the time:
"Monogamy was the rule, and a childless wife might give her husband a maid to bear him children, who were then reckoned hers. She remained mistress of her maid, and might degrade her to slavery again for insolence, but could not sell her if she had borne her husband children. If the wife did this, the Code did not allow the husband to take a concubine; but if she did not, he could do so. The concubine was a co-wife, though not of the same rank; the first wife had no power over her."
Sara was obviously observing this law when she gave Hagar to her husband Abraham for a wife to bear children to her.
Another things, Abraham also had several concubines in addition to a wife, at the same time. Should we reinstate that custom of plural concubinage as well, since we have Abraham as our example?
Tithing in Babylon
Abraham also, paid one tenth of ALL based upon the tradition already established in Babylonian law. YHWH never required anyone to "tithe" of ALL their possessions.
The standard Babylonian one-tenth tax
Hebrew was a Semitic language, related to Akkadian the lingua franca of that time. An Akkadian noun that Abraham was intimately familiar with given his Babylonian background was ‘’’’’esretu’’’’’ "one-tenth." By the time of Abraham, this was all one had to say to mean the "one-tenth tax," or "tithe." The word "tax" no longer had to be said for everyone to understand that "tithe" was meant. Listed below are some specific instances of the Mesopotamian tithe, taken from ‘’’’’The Assyrian Dictionary of the Oriental Institute of the University of Chicago’’’’’, Vol. 4 "E":
[Referring to a ten per cent tax levied on garments by the local ruler:] "the palace has taken eight garments as your tithe (on 85 garments)"
"...eleven garments as tithe (on 112 garments)"
"...(the sun-god) Shamash demands the tithe..."
"four minas of silver, the tithe of [the gods] Bel, Nabu, and Nergal..."
"...he has paid, in addition to the tithe for Ninurta, the tax of the gardiner"
"...the tithe of the chief accountant, he has delivered it to [the sun-god] Shamash"
"...why do you not pay the tithe to the Lady-of-Uruk?"
"...(a man) owes barley and dates as balance of the tithe of the **years three and four"
"...the tithe of the king on barley of the town..."
"...with regard to the elders of the city whom (the king) has **summoned to (pay) tithe..."
"...the collector of the tithe of the country Sumundar..."
"...(the official Ebabbar in Sippar) who is in charge of the tithe..."
Thus Abraham did not need to make up a new tax (a "tithe"), nor did he have to make up "one-tenth" as the amount of the tax. He did not have to make a "lucky guess," nor depend upon divine revelation to provide him with the tax and amount. Esretu was the standard word for "tax" in his homeland. The tithe, the Babylonian one-tenth tax, was simply part and parcel of the cultural baggage Abraham brought with him from Mesopotamia. He was without any doubt at all completely familiar with the concept of giving up ten-percent of whatever goods as tax.