Well, it's quite possible that they'd cancel your reservation.
If you think you're not going to be able to pull together the money in time, I'd contact them now and ask what your options are. I think there's much less of a chance that they'd cancel if you're actively working with them to make the payment, than if you just miss the date and don't contact them.
Would it be possible for a relative or friend to put the payment on their credit card, and then you could pay them back when you're able to? Disney doesn't care who makes the payment, as long as someone does.