Is it considered a forbidden act and illicit gain to sell his private water without his knowledge and keep the profit until his own water is sold to compensate for the shortage, given the employer's stinginess and refusal to increase wages?

Your taking the price of water from the employer's money to sell it for your own account, then returning it to him upon sale, is a transgression and a forbidden betrayal. You must render the trust to the one who entrusted you, for betrayal is a sign of hypocrisy. All that you have earned from selling the water must be paid to the employer, because the money and the stall belong to him, and you are his employee, and your services are his property during the period of employment. Your working for your own account or using his stall without his permission is not permissible, even if your wage is low. You can leave the job or demand a raise in your wage.