Is it permissible to add an allowance for external tasks to the invoice without the company's knowledge, in the event of their refusal to disburse it and out of fear of hardship if refused?

If the requested work falls outside what was agreed upon in the employment contract, you have the right to refuse it and request compensation. However, it is not permissible to add the compensation to the invoice without their permission, due to the Prophet's (peace be upon him) saying: "Deceit, treachery, and betrayal are in the Fire." Your acceptance to perform the work without compensation indicates that you are volunteering it. You also have the right to refuse to perform tasks outside working hours if they do not compensate you for them.