I want to report this as an error, but I’ve been wrong about these things in the past. Isn’t this an adverb? I was under the impression that adverbs never their endings to match a noun or pronoun.
I asked in an Italian WhatsApp group and they said “fina a” is an error. It should be “fino a”, Anyway in Reverso Context I found “fina a” is sometimes used, so I am guessing it’s part of a dialect, that is not standardized.