How can I record a transaction with no payment?

[lsmr]

We have dollar off coupons around town so some folks come to our ticket window prepaid for the ride and would like to use the coupon to get the dollar off. Our thought is to get them the choice of a cash back.
Is there a way to have a single transaction of a credit without a sales already started in the Square system?

[Helen]

Hello @lsmr! There isn't a way to apply a discount to payment that has already been completed. A discounts can only be added to payment at the time the payment is processed. 

Or were you looking for something else? Do your customers buy a bundle of tickets or buy tickets in advance and then find out about the coupon?