Gen 15:6 is completely ambiguous in masoretic manuscript. Any translation that does not preserve this ambiguity is in fact an interpretation.
The verse has two third person verbs, "he believed" and "he accounted", the second with a direct object pronoun "he accounted it/him", and one proposition phrase with the explicit third person pronoun "him", "righteousness for him".
If we assume that there are only two possible referents for the third person for the declensions and pronoun, God and Abram, and the object of "accounted" is "it", referring to the faith rather than either God or Abram, then there are only eight possible readings for this verse, including such readings as "God believed in God and God accounted it to God for righteousness" and Abram believed in God and Abram accounted it to Abram for righteousness.
Eliminating the obviously silly readings, we are left with two:
- Abram believed in God and God accounted it to Abram for righteousness
- Abram believed in God and Abram accounted it (God's promise) as righteousness
In context, the first reading is the simplest. God made Abram an outlandish promise and Abram believed it and therefore God counted this belief as righteousness on the part of Abram.
The objection to this reading raised by the RAMBAN and the Jewish rationalist school influenced by Aristotle is that the simple reading presents us with an unacceptable anthropomorphism - God is represented as thinking and keeping tabs on Abram, checking up to see if Abram is going to swallow the promise. Or maybe that God was pleasantly surprised by Abram's belief. However, the alternative reading leaves open the possibility that without having received the promise, maybe Abram wouldn't have though God to be righteous, i.e. "I'll believe in You if I get what I want".
Apropos the OP link, I think that trying to either justify or negate a major religious doctrine using a single verse of this type is asking for trouble.