Hi,
You can have a simple referal policy, where in you can pay the amount depending upon the levels if you have in your company. For level 1 being the highest level certain fixed referal amount can be given , which your management can decide. You can pay this amount either after confirmation of referred employee or say after three months of completion of services of the referred employee. If the refered employee leaves before confirmation , you can put the clause of recovery of referal amount from the salary of employee to whom you have paid the amount. Your referal policy should mentioned such points clearly.
If any other information is required do feel free to write to me.
Regards,
Harshad