Here the one doing voluntary devotional service is doing without asking for anything / any reward as  they know quiet well that they do not need to ask for any reward AS THE VOLUNTARY SERVICE IS ACKNOWLEDGED BY GOD AND REWARD IS GURANTEED COMING IN MANY FORMS BY GOD ...ITS LIKE AUTOMATICALLY WILL COME TO YOU AND NO NEED FOR YOU TO ASK AND THIS YOU ALREADY KNOW QUIET WELL. 

But those who do service for money are the lowest beings as here they are making money using the name of God.

SUMMERY: NOBODY SHOULD GET PAID FOR DOING ANY FORM OF SERVICE IN ANY PLACE OF WORSHIP ..IN OTHER WORDS IN GODS HOUSE. 
Bare necessities should be provided to volunteers like :

simple accommodation 

simple clothing 

simple form of transport

simple food 

THE REST GOD WILL TAKE CARE OF AS GOD KNOWS WHAT YOU WANT AND WILL NOT LET YOU DOWN