No, you have to use the entire CS or TS credit at one time. There is no way to "save" part of a credit to use later on. If you choose not to order a dessert or beverage at the time you order your meal, you will lose that part of the credit.
At some of the food courts in the resorts that have ice cream as a dessert option, you can let the cashier know you want the ice cream when you purchase your entree and beverage. Your receipt will be marked with a time limit (about an hour) so that you can come back for the ice cream when you're ready for it. This way you won't have a melted dessert, but there's no way you can go back several hours later or go elsewhere to get your dessert.