What is the ruling on the money an employee receives in full for mission expenses, even if the hosting entity covers his accommodation and food, given that the regulations of the government entity stipulate compensating him with a quarter of the amount in this case?

If the employer is authorized to disburse the full accommodation and food allowance, even if the main entity covers the cost of accommodation and food, then there is no objection to taking it. However, if they are not authorized to do so, and the disbursement is without a contractual justification, then you have no right to take it, nor does anyone have the right to grant it, due to the transgression against public funds involved therein.