Did MG not pay for both the charger and the installation for you? I was told when I ordered in January that they would be covering the cost of all (I pay them the seai grant and claim it back, but basically its free)
It sucks but the dealer is perfectly entitled to recover the cost of the charger back from you, just lucky its only 490 and not more. If you talk to them and explain how you feel, they may (as an act of good faith) wave the cost? You don't get if you don't ask! If you cancel your order and they keep the 490 deposit it would more than likely guarantee you would never return as a future customer, but if they at least return the 500 and let you keep the charger it may not sour your experience with them for any future business (hope that makes sense). You could also ask them that if the car has not arrived by the next estimated delivery time then you get to keep both charger and depost, it puts the responsibility on dealer to push for your car as well.
As for the car, it would be madness if they didnt increase the cost of the car (when it eventually arrives) before selling it on to a new customer, any business would do this.