Is it sufficient to return the rental amount to the tenant after leasing the apartment, or must the apartment be returned to the [original] party, given that the contract stipulates no selling or leasing for 20 years?

An installment sale is a valid contract. If the seller stipulates to the buyer that the apartment is not to be sold or rented until the price, or a portion of it, is paid in full, then this condition is valid. It is permissible for the seller to stipulate pledging the sold item to guarantee his right to the installments, and it is valid for the mortgagee to stipulate to the mortgagor that the mortgaged item not be rented during the mortgage period. However, stipulating the annulment of the sale and the return of the apartment if it is rented is a void condition. If the buyer rents out the apartment, the rent belongs to him and is not to be returned to the tenant or paid to the seller.