This is an excellent example of where the source text is unclear and every translator of this passage must choose one exegetical option or the other (and, hopefully, footnote the alternative). There is no option of just translating what the text "says" in this case. We know what the Greek text says, but we don't know, for sure, what it means. In English there are few, if any, options to leave the text ambiguous between the faith of Abraham or Sarah. Besides, the author of Hebrews surely intended to refer to the faith of only one of them in this passage. So if we were to translate ambiguously, because we do not know for sure what the Greek text means (even though we know what it "says"), we would not be translating authorial intent accurately.
Sigh! This is one of those times when we wish we could go back in a time machine and ask the biblical author what he (or possibly she, for the book of Hebrews) meant.
Categories: exegesis, authorial intent, translation accuracy, Bible versions