Is it permissible to receive a sum of money in exchange for attending official work meetings that take place during working hours?

If meetings and committees are an essential part of the work and agreed upon in the contract, then it is not permissible to take a separate wage for them, unless the work regulations stipulate the permissibility of disbursing additional amounts or this is decided by someone in authority. However, if these meetings are not part of the contract or the core of the work, then it is permissible for the employee to receive an additional amount for them, unless he donates it.