Genesis 15:7 NAS95 And He said to him, “I am the LORD who brought you out of Ur of the Chaldeans, to give you this land to possess it.”
After addressing the reward for helping other believers, assuring Abram he will have a child to leave his wealth to, and acknowledging Abram’s faith while they looked at the stars, God finally answered Abram’s question, “What will you give me?”
God’s answer was a cup of bitter water that tested the sincerity of Abram’s devotion. God led him to Canaan, so he can give him land to possess. The Hebrew word translated as possess means “to occupy by driving out the previous tenants and possessing their place.” 
Abram’s faith journey had been filled with delay and disappointments. First, God told him I will make you a blessing if you come see some land. A thousand miles and fifteen years later, Abram arrived to see the land. God tells him I am giving the land to your seed. But his wife is barren.
Abram spent a year wandering in a desert trying to process why he ever came to Canaan. Then he went to Egypt so he would not starve in a famine and lived in fear while he obtained wealth under false pretenses. Pharaoh discovered the truth and rebuked him for acting badly. Then escorted him out of Egypt.
He returned to Canaan where Lot took advantage of him by choosing the best land for himself and his herds. After Lot’s departure, God appeared with good news. The land belongs to Abram and to his seed. He moved to Hebron, built another altar, and waited for the birth of his son. Nine years later God returned with bad news. Abram and his seed must drive everyone off the land God is giving them.
In response to this new information, Abram asked God a reasonable question.
Genesis 15:8 NAS95 (8) He said, “O Lord GOD, how may I know that I will possess it?”
With the help of his allies, Abram had routed four kings laden with loot on their way home from battle. Driving people off the land they have lived on for generations would not be as easy. Any notions Abram may have held that he would possess the land and live in God’s city within his lifetime are crumbling, and he wanted a guarantee. God indulged him by sealing his promises with a covenant, that would be familiar to Abram.
Genesis 15:9-10 KJV (9) And he said unto him, Take me an heifer of three years old, and a she goat of three years old, and a ram of three years old, and a turtledove, and a young pigeon. (10) And he took unto him all these, and divided them in the midst, and laid each piece one against another: but the birds divided he not.
It was a prevalent practice in many ancient nations to slaughter animals and divide them in pieces so the persons making the covenant can walk between them accepting the penalty of coming to the same end as the slaughtered animals if he broke the covenant. God condescended to follow the custom of the Chaldeans, that he might confirm His oath to Abram the Chaldean.
Abram did as God instructed, then spent the rest of the day driving away hungry scavengers. As the sun set, he fell into a deep sleep, and God revealed to Abram his place in a plan that spans generations.
Genesis 15:13-14 NAS95 (13) God said to Abram, “Know for certain that your descendants will be strangers in a land that is not theirs, where they will be enslaved and oppressed four hundred years. (14) “But I will also judge the nation whom they will serve, and afterward they will come out with many possessions.
It is a common belief that Israel was a slave in Egypt for 400 years, but that is not true. The four-hundred-year time clock started ticking in 1892 when Abram entered Canaan to live as a stranger. Jacob settled his family in Egypt under Joseph’s protection in 1677. Joseph was highly favored in Egypt. His family’s arrival pleased Pharaoh. He gave them some of the best land in Egypt to live on.
They became enslaved and oppressed after Joseph died and a new king who did not know Joseph ruled Egypt. Their oppression became severe 131 years after they arrived when Egypt began throwing their baby boys in the river to drown.
During the 400 years God referenced, they were only enslaved and oppressed about 80 years. If they had accepted Moses the first time he tried to deliver them, their affliction would have been 40 years. God waited until the generation who rejected him died before he sent Moses back to deliver them.
If I were Abram, this revelation would have been depressing. By Abram’s generation, the average life expectancy had dwindled from 1,000 years to 200. Abram is 85 years old. The possibility of living another 4 to 500 years were slim. What was the point in coming to Canaan to see land he will not live long enough to possess?
 H3423 יָרֵשׁ יָרַשׁ
yârash yârêsh yaw-rash’, yaw-raysh’ A primitive root; to occupy (be driving out previous tenants, and possessing in their place); by implication to seize, to rob, to inherit; also to expel, to impoverish, to ruin: – cast out, consume, destroy, disinherit, dispossess, drive (-ing) out, enjoy, expel, X without fail, (give to, leave for) inherit (-ance, -or), + magistrate, be (make) poor, come to poverty, (give to, make to) possess, get (have) in (take) possession, seize upon, succeed, X utterly.
 https://biblehub.com/commentaries/genesis/15-9.htm Ellicott’s Commentary for English Readers
 https://biblehub.com/commentaries/genesis/15-9.htm Keil and Delitzsch Biblical Commentary on the Old Testament.