Am I allowed to prevent the employer from deducting days from workers' salaries or to lift an injustice from one of the workers without his knowledge?

Removing injustice is an individual obligation (wajib (definition) ayni) upon whoever is capable of it, provided that it does not lead to a greater corruption, and its condition is knowing that it is indeed an injustice. In such a case, one must then exert all effort to prevent or remove it, due to the saying of the Prophet, peace be upon him: "Help your brother, whether he is an oppressor or oppressed... by restraining him." We are commanded to help the oppressed. To remove injustice or prevent it, it is not a condition to seek the oppressor's permission or knowledge, if doing so would impede the removal of the injustice. Rights are reclaimed without the permission of their owners, as stated in the Almighty's saying: (So whoever has assaulted you, then assault him in the same manner that he has assaulted you) [Al-Baqarah: 194], and as in His saying: (And if you punish [an enemy], then punish with the like of that with which you were afflicted) [An-Nahl: 126]. This is also exemplified by the Prophet's fatwa (definition) to Hind bint Utbah, allowing her to take from Abu Sufyan's wealth to feed her children in a reasonable manner, without his permission.