I said to an Aalim who teaches in a mixed school that it is not permissible.

Answer

The chap is not an aalim. He is a jaahil. The obligation of an Aalim or of any Muslim is to state the Haqq in a halaal manner. Haraam ways may not be employed to propagate the Deen. Our duty is to state the Haqq, and for this only methods permitted by the Shariah are valid. When Allah Ta'ala has made an act haraam, it is contumacious to utilize that act for propagating His pure Deen.

If someone becomes a murtad, it is ordained in his taqdeer. It never means that the muballigh should violate the laws of Allah Ta'ala in a futile attempt with a haraam stunt. The last naseehat of Hadhrat Maulana Yusuf Binnuri (Rahmatullah alayh) was on this very same subject. He clearly explained the prohibition of using haraam ways for propagating the Deen. Have you seen his article?

The baseless argument about the t-cap further confirms the jahaalat of this so-called aalim. It was the way of Bani Israaeel to follow even the erroneous and baatil fatwas of their Ahbaar and Ruhbaan. Castigating them for this evil obedience, Allah Ta'ala says in the Qur'aan Majeed: "They take their Scholars and their Saints as gods besides Allah, and also for Isaa, the son of Maryam." When it suits the nafs, these juhala molvis jump on to the bandwagon of stupid taqleed - taqleed of Ulama who are no where near to the Fuqaha whose Taqleed is Waajib. If the Head Mufti of Jamiatur Rasheed had averred that the kuffaar cap is permissible, then his fatwa is a gross error. It is not permissible to follow such blatant errors of the Ulama.