Is it necessary to write down the actual amount paid for transportation, or the maximum allowable amount as a transportation allowance, given that other expenses (such as food and drink) are not sufficiently reimbursed?

If the money is travel allowance that includes food and drink, then it covers food and drink. However, if it is only transportation allowance, then it does not cover food and drink. What was paid for transportation only must be written on the form, and the remainder must be returned to the employer because it is a trust (amana). It is not permissible to dispose of entrusted money except with the knowledge of the officials. Money obtained through deceit and fraud is forbidden and deprived of blessing. Writing something other than what was paid on the form is betrayal and lying, and lying leads to immorality and Hellfire. A man may be recorded as a liar in the sight of Allah.