Is it permissible to record fictitious revenue data for an employer who fears that the rent will be raised after reviewing it?

What you mentioned about preparing untrue data is not permissible because it involves lying, which leads to immorality. The Prophet (peace be upon him) said: "Beware of lying, for lying leads to immorality, and immorality leads to the Hellfire. A man continues to lie and strives to lie until he is written with Allah as a liar." Also, it is not free from deceit and diminishing the value of the place for its owner, and Allah the Almighty says: {And do not defraud people of their things}, and this includes every wrongful reduction of wealth.