Please can you give us a bit more detail. Did you buy on FB or on the portal? And how many gems exactly did you buy?
It's usually clear exactly what you're going to get before you confirm your purchase so I'm not sure exactly what was 'switched'.
Pixel Federation, like many advertisers, makes different offers on different platforms. This is neither illegal nor unusual.
For example, I usually get offered 375 Gems plus a ship in Android but 400 Gems plus the same ship on the portal. If I take the offer, I make sure I'm buying the best package.
If your problems persist then you will have to raise a support ticket so that PF can check the game database.
I hope this helps