They have mishandled this. While people DO lie, cheat, and steal, an attempt to return an item like this, with receipt, should definitely result in them giving you a new one.

However... you say "It's not only an insult to my moral standards but also to my intelligence: who would do something as stupid as that??)"

You're right. No one would be stupid enough to put an old filter into a box and then return it.

But, if that's the case... then how DID a used filter end up on the shelves. At some point, someone must have used the filter, put it in the box, and somehow had it put on the shelves. So, yes, apparently there are people who are THAT stupid.

So, while I still agree with you, you're not making a very good claim by implying that no one would do such a thing. Obviously, someone had.