I just looked through the general conditions (downloaded from cff website) and they don't state anything clear concerning permit or minimum income... they just say that they can "decide to give you a prepaid card instead of a credit card if they choose to do so".

I think you should phone them or pass by a cff counter and ask why they gave you a pre-paid card and not the "real" one.

Well, I have it and I also have a B permit. I can't remember what information I had to give at the time but I didn't have any trouble getting it.

Yes, call them and ask again. At least they should explain. Good luck!