An example would be helpful, for I'm not sure of all the facts and cash flows here.
This sounds like an (old) EITF 00-14 issue, which I believe is now covered by ASC 605-50 (doesn't sound like 605-25, the infamous, "Multiple Deliverable" revenue recognition standard which will likely be revised this fall/winter).
This referral credit, is it given in cash or is it a, "free" product/service or anything other than cash (including "credits" that the customer can apply against trade amounts owed to you)?
If it's cash, it's likely a reduction of the selling price/revenue (certain exceptions apply).
If it's the "non-cash," it's likely considered an expense (again, certain exceptions apply).
ASC 605-50 has like 15 different examples.