In your company there is a manager. This manager leaves and the MD takes over the tasks of this manager as there is no budget to replace this manager. Over the weeks/months that follow, the MD comes to your desk and asks you to do little bits and pieces of the work that the previous manager used to do. 5 - 10 minute jobs. Have a look at this, tell me what you think of that, can you please give me this, can you book me that and so on. Nothing major. However, they are things that are not part of your job description and you are not getting paid anything extra now that you are taking on parts of a job someone else used to do. On top of this, you are getting paid less than what you were promised when you were hired in the first place.
The question is: Should you refuse to do the work as a matter of principle? The MD has already told your direct team leader that you are only asked for 5-10 minute jobs with no responsibility so there is no reason to pay you extra. The issue is not that you are too busy to do the things requested (You have plenty of time to do the jobs) but simply that you feel already undervalued at the job you are currently paid to do and now they are adding things to it without rewarding you for it or even asking you if you are prepared to do them.
What would you do in this completely hypothetical situation?
Would the answer be different if the things you are asked to do are actually much more fun and interesting than the job you are currently doing? Would you still stick to your principle?