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?

To resolve this tension, it should be recognized that the concept of a "blessing" is not simply binary. 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 that can be augmented or reduced. 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.

Additionally, it could be that God stated His two unconditional promises that He would bless Abraham, because of course, God knows the end from the beginning, and knew that Abraham would obey Him to the degree that secured the blessing of his lineage, which was manifested in Genesis 22.