What are the authentic narrations concerning the conditions of the peace treaty between Al-Hasan and Mu'awiyah, and is it true that Mu'awiyah did not fulfill them?

Dr. Al-Sallabi favored the condition stipulated by Al-Hasan to Mu'awiyah regarding the caliphate, which was that it should be decided by consultation (shura) after Mu'awiyah. However, Mu'awiyah exercised his independent judgment (ijtihad) and deemed it appropriate to appoint his son Yazid as his successor, believing it would strengthen governance. He also took a pledge from Yazid to follow the conduct of Abu Bakr and Umar. Scholars have deemed Mu'awiyah's ijtihad to be erroneous, yet they affirmed that he would not be sinful because he was a mujtahid who would be rewarded. Ibn Abi Zayd Al-Qayrawani, Shaykh al-Islam, and others emphasized the obligation of showing respect towards the Companions, thinking well of them, and interpreting their actions in the best possible light.

Therefore, it is not permissible to accuse or disparage Mu'awiyah, because all the Companions exercised ijtihad and sought Allah's pleasure. One must refrain from discussing the disputes that occurred among them and speak of them only in the best terms. Shaykh al-Islam Ibn Taymiyyah clarified that the people of Sunnah do not exempt Mu'awiyah, nor those superior to him, from sins or errors in ijtihad, and that sins have remedies that avert their punishment. The Shaykh advised reading books that exonerate Mu'awiyah and defend him.