What is the ruling of Islamic law regarding a landlord opening a commercial business similar to that of his tenant and next to it?

The basic principle is the permissibility of opening commercial shops. However, if the lessor agrees with the shop owner not to open another shop that would harm him, then it is not permissible to violate this agreement. This commitment is binding whether it is stipulated in the contract or necessitated by custom, because Muslims are bound by their conditions, and custom is given consideration. But if there is no condition or custom, then the basic principle is the permissibility for anyone to practice business.