In order to define what is certain or uncertain about Abraham's covenant with God, we must first define the word "covenant." A covenant is an agreement between two parties. There are two types of covenants: conditional and unconditional. A conditional covenant is an agreement that requires both parties to fulfill their conditions. While an unconditional covenant is an agreement between two parties as well but only one of the two parties is required to do something. The other party is not required to do anything. .
If we look into Genesis 15:18-21, we find a description of Abraham, specifically focusing on the amount of land God promised to Abraham and his progeny. There is no stipulation within the covenant that requires Abraham of anything so it makes the covenant unconditional. If we look closely at the covenant in Genesis 12:1-3. He says he will make him into a great nation, bless those who bless him and curse who curses him. If we look from the entire story of Abraham probably the certain part of the covenant which was the first to be fulfilled was the God's promise of protecting Abraham. This can be seen in his time spent in Egypt in Genesis 10:10-17 where we see God protect Abraham's wife Sarai from the Pharaoh by inflicting widespread disease to the Egyptians. That way Abraham and his wife were able to get out of Egypt unharmed. .
However, in terms of ambiguity it would probably be God's promise of a nation and descendants this because of the length of time they have taken to be fulfilled and some of which aren't fulfilled long after Abraham has died. If we look closely at the promise of descendants Abraham gets loses faith in Genesis 15:2 where he says he is still childless and hasn't been given any children and said to God that he would have a servant will become his heir. However, God reassures Abraham by telling him to count to stars and said if he could count them "so shall his offspring be.