Wow, that text is hard to look at!
First, are your dates actual dates, but formatting like that or are they strings? I'll assume strings. First, you need the minimum selected date:
Min(Faktura)
To get that same month for the previous year, you'll need some text manipulation:
Left(Min(Faktura), 4) & Right(Min(Faktura), 3)-1
The max is similar.
Then use that for Set Analysis, something like this:
Sum({1<Faktura = {'>$(=Left(Min(Faktura), 4) & Right(Min(Faktura), 3)-1)
<$(=Left(Max(Faktura), 4) & Right(Max(Faktura), 3)-1)'}> Sales)
That's really messy, so I would suggest breaking it up into multiple variables and then referencing the variables to get something like:
Sum({1<Faktura = {'>$(vMin)<$(vMax)'}> Sales)
You may also need to mess with the quotes a bit to get everything into the proper format.
