What is the ruling on renting a shop or a house by paying an advance amount, half of which is deducted monthly from the rent, with the other half being paid in cash, and then paying the full rent after the advance payment is settled, without specifying a duration for the contract?

If the monthly rent is one hundred pounds, and fifteen thousand pounds are paid in advance, from which fifty pounds are deducted monthly, in addition to the tenant paying another fifty pounds each month, so that the monthly rent totals one hundred pounds; then, when the advance payment is depleted, the tenant pays the full one hundred pounds each month. This contract is not valid without specifying the duration of the lease, although advancing the rent or stipulating a pledge or guarantor is permissible.