store credit system

Hi,
Pls help me.
I am a newbie here.So pls dont mind if i placed it in a wrong place.

I just wnt to know if there is a system in opencart in which if i give 20 store credits to my user as a bonus for subscribing on my website.Then 20 credits are equal to $1 instead of $20.
Because $20 would be a large sum.
Thanks

20 credits = 20$