What is the legitimate legal ruling on the original owner of a shop renting his shop, which he had handed over to me for investment, at the same rent that the previous tenant used to pay, so that he can use it as a warehouse, knowing that both of us benefit and no harm befalls anyone?

If you rent a shop from its owner for a known rent, and you wish to sublet the shop during the rental period to your landlord (the owner of the shop), there is no harm in that, whether at the same initial rent, or more, or less. Some jurists are of the opinion that this is permissible even if the shop has not been taken possession of, while others believe it is not permissible except after taking possession.