You can record a payment transaction that is future dated. Go to the Debt Details page for the debt account create a manual transaction. Set both the transaction date and the billing month to the month you want the payment to be posted. When the future month arrives, the payment will show up on the debt snowball table. One thing to keep in mind is that future payments don't show up on the snowball table, only payments made for the current calendar month show up (in blue).