After the diocesan bishop has heard the finance council and the presbyteral council, he has the right to impose a moderate tax for the needs of the diocese upon public juridic persons subject to his governance; this tax is to be proportionate to their income. He is permitted only to impose an extraordinary and moderate exaction upon other physical and juridic persons in case of grave necessity and under the same conditions, without prejudice to particular laws and customs which attribute greater rights to him.
Source: Code of Canon Law (Vatican.va)