Is increasing the sale price above the price set by the manager and taking this increase considered theft?

This act is impermissible and falls under unlawfully consuming people's wealth, because you are an agent, and you must give everything you earned to the owner of the money. What is incumbent upon you now is repentance, regret, and seeking forgiveness, along along with returning what you took to the owner of the money.

Scholars have stipulated that if an agent charges more than the price specified by the owner of the money or his representative, the excess belongs to the owner of the money. Al-Shanqītī said: "And if he increases, the increase is for the principal not for his agent who has not been just."