Is it permissible for an employee to benefit from a budget allocated for official assignments, knowing that he did not undertake them, and benefits from it in appreciation of his efforts or as compensation for his limited salary?

If the work system allows someone who did not undertake the missions to receive a share of the bonuses allocated to those who actually performed them, then there is no harm in taking it. However, if your superior shows you favoritism and records information that does not correspond to reality to enable you to collect it, then it is not permissible for you to take it.