Hey @Lars Werner!
You could potentially use RevenueCat’s granted entitlement system for this: https://www.revenuecat.com/docs/promotionals
Once a reward is verified, you can make a call to the granted entitlement API endpoint with the anonymous ID the user is currently verified with - this would notify RevenueCat that this user should have access to a specific entitlement for a duration of time of your choosing and you could avoid saving or handling any information in a server.
The main caveat I can see to this solution is the these types of entitlements do not transfer if the user access your app at a later time and has a new anonymous ID generated for them. If you don’t have a log in system, this could happen quite often.
I suppose it depends on the perk that you’re be offering to users - would it be something they’d use right away or something more along the lines of “1 month free”?