What is the ruling on taking sums of money in exchange for workers remaining under the sponsorship of the company while working outside of it, or leasing them to other companies for a sum of money, or selling entry visas for exorbitant amounts?

You are not permitted to take money from the earnings of the workers under your sponsorship who are working for others, because sponsorship is a contract of facilitation (irtifaq) for which it is not permissible to take compensation. However, you can agree with the workers on a monthly wage, and if someone comes who desires their services, you can agree with him on a wage higher than what you hired them for. If the worker agrees to that, there is no blame upon you, and you can profit from the difference between the two wages. As for selling visas, it is forbidden and not permissible because it is compensation for sponsorship.