I speak quite good German but have lived in Vienna for a while before moving to Bayern, so my "Bairisch" is not really very good. Today a Bavarian said to me "Das ist für Eahna", now I thought that "Eahna" was Bairisch for "Ihnen", ie the dative of "Sie", so I was surprised to hear it used after "für" where I would have expected the accusative "Sie". I have asked several Bavarian friends and colleagues but nobody could explain it. They all just said it can be used like that and it's just Bairisch. Could someone here explain the rules for using "Eahna" in Bairisch?
As Ingmar pointed out, it is dialect for Das ist für Ihnen. The only confusing thing -- as you correctly found out -- is: Why the word Ihnen here? Well, there is also nothing to explain, just get it as dialectical answer to accusative. Für wen? Für Ihnen, although you would expect Für Sie in Upper German. In the end, it really means Das ist für Sie.
Note that this is not to be confused with the plural, lower-case sie, which in dialect is Für se.
Source: My mother tongue.