What is the ruling on buying a government apartment that the seller does not yet own?

If you bought the apartment from someone who does not own it, the sale is void, whether the seller was promised ownership or contracted under a lease-to-own system. However, if the man bought the apartment from the government on an installment plan and still owes installments, the sale is valid, unless the government prohibits its sale before the installments are paid off, in which case there is a difference of opinion.