So after Abram had been living in Canaan ten years, Sarai his wife took her Egyptian slave Hagar and gave her to her husband to be his wife.
Genesis 16:3 (NIV)
The reason I’m not convinced that Abram was meant to kill the animals that God told him to bring in Genesis 15 is because of what we read in Genesis 16.
The Midrash and Jewish commentary say that the “10 years” in Genesis 16 refers to the time after which a man may leave his wife who bears no children. If a man could not "be fruitful and multiply" with his wife, he could divorce her and find another woman.
“This was the custom” is the explanation. And while that explains the actions, it doesn’t tell us that this is what God demands. Sarai’s offer of her concubine was to appease the custom.
God can bless us, but He does so in spite of our customs, not because of them.