[مَعْنًى] The default is to give fatwa by your own madhhab. Taqī ʿUthmānī allows crossing to another madhhab in set cases only. He lists three.
📘 التقي العثماني، أصول الإفتاء وآدابه، ص ٢٤٣
[مَعْنًى] A muftī may give fatwa by another madhhab when a real need or hardship calls for it. He must know that madhhab's proofs, take its view with all of its own conditions, follow its stronger view (not an odd one), and not let the result break a clear text or ijmāʿ. He must not cross madhhabs just to find an easy answer. Taking the whole view with its conditions is what keeps it from becoming forbidden talfīq.
📘 التقي العثماني، أصول الإفتاء وآدابه، ص ٢٤٤–٢٤٩
[النَّصُّ] حَقِيقَةُ التَّلْفِيقِ فِي تَقْلِيدِ الْمَذَاهِبِ هِيَ أَنْ يَأْتِيَ الْمُقَلِّدُ فِي مَسْأَلَةٍ وَاحِدَةٍ ذَاتِ فَرْعَيْنِ مُرْتَبِطَيْنِ فَأَكْثَرَ، بِكَيْفِيَّةٍ لَا يَقُولُ بِهَا مُجْتَهِدٌ مِمَّنْ قَلَّدَهُمْ فِي تِلْكَ الْمَسْأَلَةِ
[مَعْنًى] The test is the result, not how many acts there are. It is forbidden when the parts add up to a state no madhhab accepts as valid. Picking rulings just for ease or desire, with no real need, is also forbidden.
📘 التقي العثماني، أصول الإفتاء وآدابه، ص ٢٤٩، وقرار مجمع الفقه الإسلامي ص ٢٥٨
[النَّصُّ] إِذَا صَحَّ الْحَدِيثُ وَكَانَ عَلَى خِلَافِ الْمَذْهَبِ، عُمِلَ بِالْحَدِيثِ، وَيَكُونُ ذَلِكَ مَذْهَبَهُ، وَلَا يَخْرُجُ مُقَلِّدُهُ عَنْ كَوْنِهِ حَنَفِيًّا بِالْعَمَلِ بِهِ
[مَعْنًى] This is only for a muftī grounded in the madhhab who can weigh proofs (ahl al-naẓar, أهل النظر), and only where a mujtahid has already acted on that hadith. It is not for a layman.
📘 التقي العثماني، أصول الإفتاء وآدابه، ص ٢٥٩
[مَعْنًى] If a qāḍī judges in a matter the jurists dispute (masʾala ijtihādiyya), his ruling holds and a later qāḍī may not overturn it, because the ruling lifts the dispute. It is void only if it breaks a clear text or ijmāʿ. And the order of a Muslim ruler on a disputed matter is to be followed, even against one's own madhhab view.
📘 التقي العثماني، أصول الإفتاء وآدابه، ص ٢٦٣–٢٨١