Abraham was the first instance of the first-son/second-son theme that Matthew saw. The theme says that he first son loses an inheritance and/or is fruitless and is also portrayed as being earthly. The second son is the heavenly son who gains the inheritance and is fruitful.
There are two ways that this theme is played out concerning Abraham. He is literally a second son and he is figuratively a second son.
Abram the literal second son
Genesis appears to give Abraham pre-eminence as the first born in the listing of the sons of Terah. This itself is a riddle, since he is the second son, as the numbers will prove out.
- Ge 11:26 And Terah lived seventy years, and begat Abram, Nahor, and Haran.
Terah was 70 years of age when Abram was born
- Ge 11:32 And the days of Terah were two hundred and five years: and Terah died in Haran.
Terah dies when he was 205.
- Ge 12:4 ¶ So Abram departed, as the LORD had spoken unto him; and Lot went with him: and Abram [was] seventy and five years old when he departed out of Haran.
Upon Terah's death Abram leaves Ur; he was 75, 205-75=130. Terah was 130 years old when Abraham was born, and 70 years when Haran was born.