Is it permissible for the seller to stipulate that the buyer not demand the remaining rent amount from the tenant, which the seller had already collected in advance, and is that considered deceit?

Leasing is a binding contract, in which the lessor owns the price and the lessee owns the usufruct. It is not invalidated by the sale of the leased asset if the purchaser is aware of the lease. The lessee who purchases the leased asset becomes the owner of both the usufruct and the property, and there is no contradiction in this. The new purchaser does not have the right to demand rent for the remaining period, and the sale is valid. If the purchaser is unaware of the existence of the lease, then he has the option to annul the sale.