They should pay their own dues or have their religious organisation pay for them.   
I quote Article 27 of the Law
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Under the terms of this Act, registered churches and other religious communities may, for their religious servants who are citizens of the Republic of Slovenia with permanent residence in the Republic of Slovenia and for whom the profession of religious servant is their sole profession, apply, on the basis of their authorisation and on their behalf, for the right to receive earmarked State financial assistance from the State budget to cover social security contributions in the amount of the following types of social security contributions on a base of at least 60 per cent of the average salary for the month preceding the month in which the insurance base is determined: for compulsory pension and invalidity insurance (contribution by the insured person) and for compulsory sickness insurance (contribution by the insured person). This right cannot be acquired by religious ministers employed under the provisions of.