Sander The solution is:
Give 10 credits each (inviter and invitee). And for using bunq card every week for 3 months, they can exchange credits to € after 3 months. (1 credit = 1€).
For example:
Inviter invite on April 19 his first invitee and his 2nd on April 25.
After 3 months his first 10 credits on July 19 he can exchange it to €. And his 2nd on July 25. And these conditions is also for the invitee.