If you want it configured as a tender then follow the Samba Card tutorial for v3.
Here is a post containing a link to the original post and also an export file.
As @JTRTech mentions, I think that this ‘credit’ should be applied as a discount as no money has been exchanged.
You can see my discussion with QMcKay and how this was achieved here Loyalty Card Credit as Discount vs Tender - #8 by QMcKay