Is it permissible to falsify lease agreements to obtain a housing allowance higher than what is due, under the pretext of increasing one's salary to cover hospitality expenses and a presentable appearance?

If the state only covers the actual rent expenses for housing, then it is not permissible to defraud it by submitting fictitious rental contracts, because this constitutes fraud, betrayal, and consuming people's wealth unjustly.

The need for sufficient expenses is not a justification for this, as contracts are based on mutual consent. If the amount provided is insufficient, the concerned party must be informed to raise the allowances. If they agree, then that is good. Otherwise, the person has the choice either to continue working with the given amount or to seek another job that suffices their needs.