The substance produced by the human liver that is stored in the renal medulla to maintain the desired osmolarity is

Ammonia produced by metabolism is converted into urea in the liver of humans. It is released into the blood which is filtered and excreted out by the kidneys. Some amount of urea is retained in the kidney matrix to maintain a desired higher osmolarity in the renal medulla. This facilitates reabsorption of more water in the collecting ducts resulting in the excretion of concentrated urine.