secret rebate

Definition from Nolo’s Plain-English Law Dictionary

A kickback of money by a business to a "preferred" customer, not offered to the public, or by a subcontractor to a contractor, but not shown on a job estimate. Both practices are illegal in most states as unfair business practices and may result in criminal penalties or refusal of a court to enforce a contract (written or oral) in which there is such a secret rebate.

Definition provided by Nolo’s Plain-English Law Dictionary.