Something to think about.
Genesis 15.6 reads:
And he believed the LORD; and the LORD reckoned it to him as righteousness. (NRSV)
וְהֶאֱמִן בַּיהוָה וַיַּחְשְׁבֶהָ לּוֹ צְדָקָה (MT)
καὶ ἐπίστευσεν Αβραμ τῷ θεῷ καὶ ἐλογίσθη αὐτῷ εἰς δικαιοσύνην (LXX)
The NRSV reads the subject into the second clause: YHWH does the reckoning of righteousness to Abraham. The LXX already suggests this interpretation by specifying Abraham as the subject of the first clause and therefore the referent of the αὐτῷ in the 2nd.
But the MT says none of this. Bertil Albrektson ('A Disputed Sense in a Covenant Context: On The Interpretation of Genesis 15:6', in A.D.H Mayes and R. B. Salters (eds), Covenant as Context: Essays in Honour of E. W. Nicholson (Oxford: OUP. 2003), 1-9) suggests that the natural sense is that Abraham 'trusted YHWH and considered it (i.e. YHWH's promise) reliable.
How does that reading affect our understanding of Paul's use of the quotation at Romans 4.3 and Galatians 3.6?