Is it permissible for my friend to demand the earnest money from the person who backed out of renting the apartment in his place, on the grounds that they had an agreement and he stopped searching for an alternative?

The money paid to the landlord serves as a pledge to guarantee his right throughout the rental period, and it is returned to the tenant afterward.

The tenant does not have the right to terminate the contract before the end of the period unless the landlord agrees to bring a replacement tenant.

The original tenant may rent the property to another tenant after taking possession of the usufruct.

If the second tenant terminates the contract, he is obligated to pay the agreed-upon rent for the entire period and is not required to provide a pledge. However, if he does not terminate, he is only required to pay the rent on its due dates.

According to the majority of scholars, a pledge is not binding until it is seized and delivered. However, the Malikis hold that it becomes binding merely upon the contract, and the landlord has the right to bring the matter before a judge to compel the tenant to provide the pledge or to pay the full rent.

Neither the first nor the second landlord is permitted to benefit from the pledge, based on the saying of the Prophet, peace and blessings be upon him: "The pledge is not locked against its owner; its produce is for him, and its liability is upon him."