Imagine we drop a 1000 kg object from 5 km height. When it reaches the ground, we obtain potential energy. Then we convert the object (mass) into energy, we transfer the energy back to 5 km height (assuming no energy is lost) then we convert back the energy into mass, then we repeat the process. Drop the object, obtain potential energy, convert to energy, transfer it to 5 km height, convert it back to matter. I understand this is hypothetical since no technology we have today allow us to perform this. How conservation of mass-energy get along with this?
*Energy can actually be converted into matter https://www.forbes.com/sites/paulrodgers/2014/05/19/einstein-was-right-you-can-turn-energy-into-matter/