Is the beggar burdened with sin if he donated the money he obtained for himself and his colleague, after their salaries were withheld due to the injustice they suffered at the company they work for?

What the manager does, by way of injustice and unlawfully consuming people's wealth, requires him to absolve himself of these wrongs. What you took in return for your work is your right, and it is permissible to take it without his knowledge if he delays in giving you your right; this is known as the issue of al-Zafar (taking one's right when the oppressor denies it). However, if the amount exceeds your right, you must repent and return the money to its owner indirectly.