Is it obligatory to donate the entire amount of a bonus mistakenly disbursed, or is it permissible to benefit from part of it and donate the remainder?

If you are not entitled to the amount according to the regulations, you must dispose of it by returning it to the responsible authority, or by giving it as charity to the poor and for the benefit of Muslims. It is not permissible for you to take any of it for yourself. The institute's inability to reclaim it, or the employee's statement that you are free to dispose of it, holds no weight. That does not make the amount permissible for you.

It is stated in the fatwas of the Permanent Committee that: "It is not permissible for your direct supervisor to issue you a secondment decision when you have not actually undertaken this secondment; as this involves lying, treachery, and violating the regulations established for the entitlement to the money associated with it. Your taking money for this secondment is taking it without a legitimate right. If your workplace does not accept the return of the money resulting from this secondment, then you should spend it on various acts of righteousness."

The Committee was also asked about a responsible person who arranged a secondment for another person during his vacation without assigning him any work. Their answer was: "It is not permissible for any person to take anything from the state's money, or from anyone else for whom he works, without right, nor to authorize that for others, if he knows that his authorization is contrary to what is right. This is supported by what Abu Dawud narrated in his Sunan, on the authority of Abdullah bin Buraydah, from his father, from the Prophet (peace be upon him), who said: «Whoever we employ for a task, and we provide him with a provision, then whatever he takes after that is ghulūl (embezzlement).» Therefore; it is not permissible for this person to take the money allocated to him from the secondment he did not undertake; because he has no right to it, and it involves injustice to others, and infringement on the rights of others without a legitimate right. Similarly, the salary is not permissible for him for the period during which he left work without a legitimate justification."

It is also stated in the fatwas of the Permanent Committee: "If the employee is not excused for days of absence, then he must return the salaries for the days of absence to the entity he works for. If that is impossible, or if it would cause him great harm, then he should give it as charity to the poor."