In Genesis 12, God tells Abraham that he will bless him, exalt him, and bless all nations through him:
Genesis 12:1-3
1 Now the Lord had said unto Abram, Get thee out of thy country, and from thy kindred, and from thy father's house, unto a land that I will shew thee:
2 And I will make of thee a great nation, and I will bless thee, and make thy name great; and thou shalt be a blessing:
3 And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.
Notice that there are no requirements or conditions to what God says.
Later on, before Abraham has either Ishmael or Isaac, God reiterates this promise, also without mentioning conditions:
Genesis 15:4-5
4 And, behold, the word of the Lord came unto him, saying, This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir.
5 And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be.
However, after Abraham offers Isaac in Genesis 22, God says:
Genesis 22:16-18
16 And said, By myself have I sworn, saith the Lord, for because thou hast done this thing, and hast not withheld thy son, thine only son:
17 That in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore; and thy seed shall possess the gate of his enemies;
18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.
Notice above that God says that Abraham will be blessed because he obeyed. How does that make sense, when God already promised to bless him, without any conditions, twice before? If he wouldn't have obeyed, would those earlier promises have been invalidated?
Firstly, we can recognize that the concept of a "blessing" is not simply a binary thing. There is a massive gradient on which God can choose to bless someone. There are certain ways in which God blesses everyone (Matthew 5:45), and He adds or subtracts from there. So, in light of that, the simplest solution is that God is rewarding Abraham with a more abundant blessing, because of his obedience, which he would not have gotten had he not obeyed.
Secondly, the idea that the first two promises hung in the balance based on whether Abraham obeyed would simply render God a liar. God told Abraham that He would bless him, and his children, and not a single caveat was mentioned. Since it's arguing from silence to suggest that God gave Abraham conditions that weren't recorded, we can assume He didn't. That means that if God didn't follow through, He would have lied to Abraham - something God cannot ever do (Titus 1:2, Hebrews 6:18). God will never renege on a promise or commitment (Numbers 23:19: 1 Samuel 15:29, Romans 11:29).
God tested Abraham's faith, and promised to bless him more abundantly as a result of his obedience, which added to the blessings that God already promised him unconditionally at earlier points in his life.