Fatwa of Sheikh Fawzān ḥafidhahullāh.

Question: O Esteemed Sheikh may Allāh grant you success, this questioner from Britain says: we have some masjids in Britain that bring someone upon the manhaj of the Sunnah to teach people Tawheed or give a lecture concerning Tawheed, but the following day they bring a Sufi or an Ash'ari who teaches people Sufiyyah or Ash'ariyyah. Our question is, is it permissible for the Sunni da'ee to teach in these types of masjids? Answer: No. If they are going to mix between him and the innovator or the misguided one, then it's not permissible for him to do so; because they are using him as a screen, (they will tell people) 'look we teach tawheed, and we allow lectures on tawheed.' This is not right. This is mixing truth with falsehood. He should make a condition upon them that, 'I will teach you, or I will lecture in your masjid with the condition that you do not bring anyone who will oppose what I call you to or what I tell you.' Source: https://t.me/madrasatuna/2518