If a member already has a recurring discount added to a plan they are on (or doesn't), you have the option of adding a single use discount that will apply to the next billing cycle. This can be good for things like referral discounts or any time you want to discount their next bill.
Navigate to their member profile page.
Select 'Change Amount/Discount from the 3 dots context menu on the plan you want to add this discount to.
Click 'Add one-time discount'
Enter amount in field.
Hit Tab key or click out of the field box to view discount.
Save Changes to apply the discount to plan.