“For if the inheritance be of the law, it is no more of promise: but God gave it to Abraham by promise.” God did not PROMISE Abraham the inheritance. He GAVE the INHERITANCE to him “by promise”. - Gal. 3:18
And, WHAT did God give Abraham? (I.e. What is the INHERITANCE?) It is whatever God Promised Abraham in that Covenant (which was “confirmed before of God in Christ”, which could not be “annulled by the law” – Gal 3: 17). The covenant spoken of is the covenant of grace and redemption. It is the Everlasting Covenant of Grace made between the Persons of the Godhead. It is a covenant of Eternal Salvation with all its blessings and benefits. Abraham was GIVEN the INHERITANCE. He was GIVEN Eternal Salvation. Like all the elect, Abraham had ALREADY been given “grace” (all the benefits and blessings of salvation “before the world began” (II Tim 1: 9).
The “giving” here is as a POSSESSION. In time (in Regeneration), God enabled Abraham to take POSSESSION of (by Promise and by faith) what He had ALREADY given him “in Christ before the world began”. According to Romans, Abraham was “fully persuaded that what God had promised, He was able also to perform.” What had God Promised Abraham? Salvation? That might be your immediate answer. But, think about it. God did not Promise Abraham salvation. He PROMISED him a seed, a seed through whom “all the nations of the earth would be blessed”. He promised him a Savior who would “finish the transgression, make and end of sin, and bring in everlasting Righteousness”. He promised him a Savior who (by His obedient life and sacrificial death) would work out all the details of Eternal Salvation. Christ came in time to meet all the conditions and requirements of that salvation His elect people had ALREADY been GIVEN in Him “before the world began”. That is what Abraham saw in God’s Promised Seed. And, Abraham believed God (I.e. He was given faith in the Promised Seed). And, that faith gave evidence that God had imputed to Abraham for righteousness what Christ would accomplish in time.