Is the owner or the one managing their properties entitled to demand a commission or administrative fees from tenants?

The landlord or his agent is not entitled to request an increase in the agreed-upon rent before the contract expires. After the contract expires, if the increase (commission) is a lump sum paid once, and the lease is for a fixed term (wajiba), then it is permissible to add it to the contract if both parties agree. However, if the lease is monthly (mushahara), this lump sum increase is not permissible due to the resulting ambiguity in the rent. If the increase is a fixed amount taken every month, it is permissible absolutely, whether the lease is for a fixed term or monthly, as there is no ambiguity.

As for the agent taking this commission for himself, it is not permissible unless it belongs to the landlord and the landlord has authorized him to take it. Otherwise, taking it constitutes deception and consuming people's wealth unjustly, based on the Almighty's saying: "O you who have believed, do not consume your properties among yourselves unjustly."