How soon do you board? Difficult to cancel a ticket once you've flown the flight! 
But if it is an error on their part, as suggested above, I'd ask them to waive the difference as a good will gesture. If they still refuse, I think the right thing to do would be to pay the correct amount of points, although I'd also ask them for a bit of leeway on the timing so you can transfer points from a credit card/organise a family transfer/beg/borrow/steal the necessary points to avoid paying the extortionate top-up rates.
