You can only sell full contracts. The deed itself is restricted with verbiage that reserves the right to split up the interest in the property to Disney.
If you buy 200 points in two 100-point contracts, you can sell one of them later.
That's a more complicated purchase, though.
You'd probably be better off buying one 200-point contract, and if you feel like reducing your points later, buy a smaller contract and sell your existing one.
Alternative, you could just keep the existing contract and rent the points out that you don't want to use. Lots of options.