No. Coins and rewards are not prepaid by default, though you can choose to prepay for rewards under Rewards Manager, Funding. Coins are free to give and receive.
Who can use this: Workspace Owners and Admins on the Pro and Enterprise plans, or on any plan during the free trial. Works the same for Slack and Microsoft Teams workspaces.
For an overview of the whole feature, read the Rewards Guide.
You will only be charged when a Workspace Owner/Admin approves a reward redemption request for gift cards, prepaid cards, monetary options, premium gifts, 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.