Paul writes in Romans 4.
For what does the Scripture say? “Abraham believed God, and it was counted to him as righteousness.” Now to the one who works, his reward is not counted as a gift but as his due.
Paul, as N. T. Wright has argued is probably not referring to abstract wages, but to the specific reward in Genesis 15 to which he refers as he writes of Abraham
After these things the word of the LORD came to Abram in a vision: “Fear not, Abram, I am your shield; your reward shall be very great.”
Both Abra[ha]m and YHWH understand this reward to be an inheritance in people and land. Thus Paul argues this reward includes the “cosmos” and a people who are both his own descendents in the covenant of circumcision and all the uncircumcised believers in all the nations (“Gentiles”).
Paul is then arguing that the inheritance that Abraham was promised was not earned but rather inherited by trusting God’s promise.
This raises the further possibility that Paul is arguing that the promise to Abraham has now come true in Christ. That certainly seems to be how Romans chapter 4 concludes:
But the words “it was counted to him” were not written for his sake alone, but for ours also. It will be counted to us who believe in him who raised from the dead Jesus our Lord, who was delivered up for our trespasses and raised for our justification.
But this might be also the way Romans 4 is introduced. Consider how Genesis 15 reads. God spells out to Abraham how his seed will come to inherit the promised land:
Then the LORD said to Abram, “Know for certain that your offspring will be sojourners in a land that is not theirs and will be servants there, and they will be afflicted for four hundred years. But I will bring judgment on the nation that they serve, and afterward they shall come out with great possessions.”
This predicts and communicates to later readers the entire Passover story, with the displayed blood on the doorway turning away the destroyer so that Israel could be redeemed from Egypt.
And this is precisely what Paul has already alluded to at the end or Romans 3:
For there is no distinction: for fall have sinned and fall short of the glory of God, and are justified hby is grace as a gift, through the redemption that is in Christ Jesus, whom God put forward as a propitiation by his blood through faithfulness.
I don’t think Paul could write those words without intending to remind us of the Passover night and the Exodus when Israel, along with a multitude of believing Egyptians left the house of bondage.