Homer has εἰ and the indicative with οὐ ( cross12 times) when the subordinate clause precedes the main clause; but usually εἰ μή, when the subordinate clause follows. Thus, εἰ δέ μοι οὐ τείσουσι βοῶν ἐπιεικἔ ἀμοιβήν, δύ_σομαι εἰς Ἀίδα_ο
a. The Homeric εἰ οὐ with the indicative has been explained either as a retention of the original use, μή with that mood being an extension through the analogy of the subjunctive and optative; or because οὐ went with the predicate, whereas μή was closely attached to εἰ.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].