Can they make my DP do something that isn't part of his job description?

DP is the assistant fine wine manager at our store but has learnt that he has to do some register shifts as another manager is under the impression our store manager doesn't think he works hard enough.

We are angry/upset because DP stepped down from Service manager because he didn't want to work on registers or be a part of that he wants to work in fine wine and extend his knowledge in wine.

DP is now going to call our area manager and the fine wine specialist and request to move stores which means i have to cut hours at work so we lose money which right now we can't afford.