MalloryCat it could be that the person that told you this is loosely equating B permit with longterm or permanent resident and therefore jumping to the conclusion that you don't get the expat allowance because you are not an expat. But see my post above. It isn't so black and white and it ain't necessarily so. There are still arguments for getting the expat allowance even if you are not temporary expat and you should go for them anyway.

By the way, if you really are there on a temporary basis, it would be better to have a return date in your assignment contract - for the avoidance of doubt (it can always be extended).