Is the additional amount of 2000 Saudi Riyals, resulting from the difference in airfare between the family's early travel date and the date of receiving end-of-contract dues, considered permissible (halal) and rightfully owed to the employee, taking into account his bearing the expenses and hardship of the family's early travel and the risks of changing the reservation?

If the employer grants the employee a specific amount for travel tickets as ownership, regardless of the actual cost, then this amount belongs to the employee and he is free to dispose of it. However, if the employer pays for the tickets according to their actual value or entrusts the employee with purchasing them and reimburses him for their cost, then it is not permissible for the employee to take an amount exceeding the real value of the ticket, otherwise he would be sinful, as Allah Almighty says: "And do not consume your properties among yourselves wrongfully." He must repent and return the excess amount. The questioner should determine his situation based on these two categories, and if the matter is unclear to him, he should refer back to the company.