Doctrine of Exchange

The Doctrine of Exchange is a central tenet of Scientology, which dictates that for spiritual well-being, "anytime a person receives something, he must pay something back" and balance "inflow" with "outflow".   The Church of Scientology has presented this as the reason some of its services, such as auditing, its central practice of Scientology, must never be given away, but must be paid for. 

Quid pro quo transactions are prohibited in tax-exempt organizations, and the Church of Scientology has argued in its requests for tax exemption that Scientology courses must have fixed fees because of this religious doctrine. 

Exceptions
Not all services fall under the Doctrine.   The Church has identified some services as examples of services where "no donation is expected from members":
 Listening to lectures, whether from fellow parishioners or playing L. Ron Hubbard’s lectures on tape
 Reading Scientology scripture in the Church library
 Meeting with fellow parishioners
 Receiving counseling (but not auditing) from a Scientology chaplain
 Attending Sunday services, sermons, weddings, christenings or funerals

References

Category:Scientology beliefs and practices
Category:Scientology-related controversies
Category:Taxation in the United States