What is the expiation for mistakenly taking casual leave, and is it obligatory to donate its value or compensate for it from one's accrued leave balance?

You can refer to the competent authorities in your workplace to correct the error of taking leave without it being deducted from your balance. They may absolve you, or deduct it from your salary, or count it as part of your annual or incidental leave. If you are unable to inform your workplace, then return the value of five days' salary indirectly. It is not sufficient to leave five days from your balance without the permission and consent of the workplace.