In both Genesis and Romans there is no doubt that the subject was Abraham. Moses and Paul could not be talking about another person here other than Abraham. There are only 3 characters here and one of them was narrating, and that is, Moses. By eliminating Moses we have God and Abraham. Now, if we say that the pronoun “HE” in Genesis 15:6 was Abraham then that verse will come out like this:
Ge 15:6 And he/Abraham believed in Jehovah; and He/Abraham reckoned it to him for righteousness.
Instead of:
Ge 15:6 And he/Abraham believed in Jehovah; and He/God reckoned it to him for righteousness.
This is not the way the Hebrew language works. Whenever the personal pronoun is used it is always in reference to something previously said. The subject of the text is speaking of Abraham…not God. But you have to read the whole verse in context for it to make sense.
5And 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.
6And he (Abraham) believed in the LORD; and he (Abraham) counted it to him (YHVH) for righteousness.
7And he said unto him, I
am the LORD that brought thee out of Ur of the Chaldees, to give thee this land to inherit it. Gen 15:5-7
Abraham considered YHVH righteous and that He would fulfilled the oath that He just offered. As I already explained, this interpretation is proven by this following text.
"Sojourn in this land, and I will be with you and bless you; for to you and your descendants I give all these lands, and I will perform the oath which I swore to Abraham your father. And I will make your descendants multiply as the stars of heaven; I will give to your descendants all these lands; and in your seed all the nations of the earth shall be blessed;
BECAUSE Abraham obeyed My voice and kept My charge, My commandments, My statutes, and My Laws." Genesis 26:3-5