You're only hitting half the points here. 1) Yes, a referral bonus is optional 2) It no longer becomes optional once you make the offer to your employees. At

You're only hitting half the points here. 1) Yes, a referral bonus is optional 2) It no longer becomes optional once you make the offer to your employees. At this point you have established intent; especially if it is provided in writing. You cannot take it back once the other party involved has upheld their end of the arrangement.