What happens to unused campaign budget?
Short answer
If fewer views are verified than your budget covers, the unused part of the budget comes back to your balance. You are billed only for verified views.
Your budget sets the maximum you can spend, not a fixed bill. Billing is based on verified views multiplied by your rate, so any part of the budget that was not used for verified views is not charged.
That returned amount stays on your balance for your next campaign.
This is also why your risk on Pomera is capped by the budget you set. See can I set a maximum budget.