Is it permissible to donate the salary of a month in which one was absent from work?

You must return the salary you received for the month you were absent. It is not permissible for you to dispose of it through charity or spending as long as you are able to return it to your employer.

If you are unable to return the money to your employer and there is no way to do so, then it is permissible for you to give it as charity to the needy or spend it on acts of righteousness and public welfare.