charge for service not authorised

My watch was booked in for a repair done under warranty. When i booked it in i specifically ask them to call me should there be any charges even though the warranty period is still valid. No communication was made with me until the watch was fixed and ready for collection. They made contact with me via text message. I arrived back at the store later that day only to be told that a charge of R220 was to be made for a battery replacement which had been carried out with my authorisation. They will not return my watch until this has been paid.