I remember reading that there was a letter (Geonic, I believe) advising his correspondent to look at the interpretation of a local Christian clergyman of a pasuk; that his was better than the one that the correspondent had. It’s been brought as an example of שמע האמת ממי שאמרה. Can anyone help find that source?
Not a letter from the Gaon, exactly, but a personal instruction to that effect.
The story is that one day, in the Yeshiva in Baghdad, they were debating the meaning of the phrase שמן ראש אל יני ראשי (Tehillim 141:5). Rav Hai Gaon instructed R' Matzliach ibn al-Bazak, later a dayan in Sicily and one of the teachers of R' Nassan Baal HaAruch, to ask the local (Nestorian) Catholicos for their interpretation of the verse. When R' Matzliach demurred, Rav Hai told him sharply that our holy predecessors never had a problem asking members of other religions, or simple shepherds or cowhands, for the meanings of words in the Torah - and R' Matzliach then went and did as he was told.
R' Matzliach in turn tells the story in a letter to R' Shmuel Hanaggid. The relevant excerpt is in Ginzei Kedem 3 (1925): 67.
Note that it's not that Rav Hai tells R' Matzliach that the non-Jewish interpretation is better, just that we need to know it.