conditioning for moneys to be paid - Public forum - Planning Advisory Service (PAS)

Just a quick question. Does anyone know the legal reason, either in terms of statute or case law, why a condition on a planning permission cannot ask for money to be paid by a developer. Provided in meets the tests for a conditions prescribed by circular 11/95 and case law, and has a suitable reason, im struggling to find a reason why it is not allowed. At least from looking though some of my law text books. Any help woule be much appreciated.