This might be considered a breach of fiduciary duty. The Cornell Legal Information Institute, for example, defines the Duty of Loyalty as:
The duty of loyalty stands for the principle that directors and officers of a corporation in making all decisions in their capacities as corporate fiduciaries, must act without personal economic conflict. The duty of loyalty can be breached either by making a self-interested transaction or taking a corporate opportunity.
It appears that using his dual roles as the company’s landlord and its manager to negotiate a better deal for himself at his employer’s expense, would breach that.
It’s difficult to see how the manager/landlord would be able to avoid taking advantage of the situation, even if he tried. For instance, would the company be able to replace him as manager if they knew he would then evict them in his capacity as landlord?
The purchase itself might breach rules as well. This might, depending on circumstances, fall under the doctrine of a corporate opportunity:
". . . a corporate officer or director may not take a business opportunity for his own if: (1) the corporation is financially able to exploit the opportunity; (2) the opportunity is within the corporation's line of business; (3) the corporation has an interest or expectancy in the opportunity; and (4) by taking the opportunity for his own, the corporate fiduciary will thereby be placed in a position inimicable to his duties to the corporation. The Court in Guth also derived a corollary which states that a director or officer may take a corporate opportunity if: (1) the opportunity is presented to the director or officer in his individual and not his corporate capacity; (2) the opportunity is not essential to the corporation; (3) the corporation holds no interest or expectancy in the opportunity; and (4) the director or officer has not wrongfully employed the resources of the corporation in pursuing or exploiting the opportunity. Guth, 5 A.2d at 509."