What is the concept of fiqh for the da'iyyah (Islamic caller)? And what is the difference between a judge (qadi) and a market inspector (muhtasib)?

The jurisprudence of the Da'ee (caller to Islam) is the knowledge that clarifies the rulings and regulations a Da'ee needs for his call to be legitimate and based on insight. It encompasses the characteristics of the Da'ee, the conditions of the one being called (Mad'u), the means of Da'wah, and the prioritization of its objectives. As for the judge (Qadi), he is the one who rules between litigants, and it is not permissible for the unqualified to undertake this role, as the Prophet (peace be upon him) said: "Judges are three: one is in Paradise and two are in Hellfire. A man who knew the truth and judged accordingly is in Paradise. A man who judged between people out of ignorance is in Hellfire. And a man who acted unjustly in judgment is in Hellfire." The Muhtasib, linguistically, is one who seeks reward. Technically, he is one who enjoins good and forbids evil, and monitors people's actions to encourage them to righteousness.