The Prophet ﷺ said: Conciliation between Muslims is permissible. The narrator Ahmad added in his version: "except the conciliation which makes lawful unlawful and unlawful lawful." Sulayman ibn Dawud added: The Messenger of Allah ﷺ said: Muslims are on (i.e. stick to) their conditions.
قَالَ رَسُولُ اللَّهِ ﷺ الصُّلْحُ جَائِزٌ بَيْنَ الْمُسْلِمِينَ زَادَ أَحْمَدُ إِلاَّ صُلْحًا أَحَلَّ حَرَامًا أَوْ حَرَّمَ حَلاَلاً وَزَادَ سُلَيْمَانُ بْنُ دَاوُدَ وَقَالَ رَسُولُ اللَّهِ ﷺ الْمُسْلِمُونَ عَلَى شُرُوطِهِمْ