Let's say you wanted to convince some people that Abraham's example (of simply trusting the Lord's promise) was not our example of how we are saved today. If you were a tricky thinker, how might you convince people that God intended us to earn eternal life by obeying commandments? Well, one way you might try to fool people into legalism is to say that Moses' covenant replaced Abraham's covenant. Abraham's covenant was a "prior dispensation", or something like that.
But Paul says no to that idea. His example is that of a normal earthly contract or treaty. Once a human contract has been duly signed and sealed, no one can come moseying along, days, months, or years later, and simply change it. In the same way, God made promises to Abraham and his seed, and no one but God could change it.
Paul makes a brilliant grammatical observation. He points out that God made his promises, through Abraham, to Jesus Christ. Jesus is the ultimate seed of Abraham, Matthew 1:1. God in Genesis 12:7 didn't use a plural noun ("seeds"), which, if He had done, would make it impossible to fix the promise on just one inheritor. Instead, God used a singular noun ("seed"). That singular Seed is Jesus of Nazareth. The Messiah is the inheritor of Abraham's covenant.
Christ will become king of the land. He will also have spiritual offspring (see the promises in Isaiah 53:10, and the quotation from Psalm 8 cited in Hebrews 2:13). He received the blessing of eternal life, when God raised Him from the dead. Through Abraham's covenant, God the Father made promises to.... God the Son! The members of the Trinity are not going to cancel their own agreement that they made between...themselves!
So Abraham's Covenant cannot be annulled or changed. God pledged the inheritance on the basis of promise. God did not require Abraham to do a good work, in order to receive the promise. In fact, God put Abraham to sleep, and walked down the trail of consecrating blood all by Himself (Genesis 15).
No one mediated God's promise to Abraham. God appeared to Abraham directly, as Abraham was in a trance. But Moses was the mediator between the Lord and the people. No angel communicated God's promise to Abraham. God Himself communicated it to Abraham. But angels communicated God's law to Moses. Abraham's covenant was a superior covenant to Moses.
Abraham's covenant is a timeless covenant, and was made by God the Father with God the Son, through Abraham. It isn't that the blessing of salvation was by faith alone for 430 years, and then, once Moses came down the mountain, God changed salvation into a law-keeping system. The blessing of salvation came by faith in God's promise back in Abraham's time, it still came by faith during Moses' time, and it comes by faith to this very day.