No! Coins/Rewards are not prepaid. Coins are free to give and receive.
You will only be charged when a Workspace Owner/Admin approves a reward redemption request for gift cards, prepaid card, or charitable donations.
Which means Owners/Admins are in control of how much they are charged and when they are charged. They can wait to approve rewards or auto-approve them.
Note: There is no cost associated with custom company rewards because companies are responsible for the delivery/fulfillment of the reward.