Is it obligatory to return the rent amount that was deceitfully taken from the landlord who violated the terms of the contract, and if its return is obligatory, is it permissible to donate it to charities with the intention that the reward goes back to the landlord in case it is impossible to reach him?

The landlord must fulfill what was agreed upon. If the breach of the condition affects the value of the house's utility and has not been consented to, then you have the right to "Arsh al-'Aib" (compensation for the defect), which is the difference in rent between the house being defective and being sound, estimated by experts.

However, the majority of scholars hold that you only have the option to annul the contract or keep it without "Arsh."

The fatwa (definition), therefore, is that you return the full amount mentioned to the lessor or their heirs. If it is impossible to reach them, then donate it to the general welfare of Muslims.