In 1128, the atabeg of Damascus Toghtegin dies, leaving his son Buri along his vizir Al-Mazdaghani in power.
Since the Assassins were massacred in Aleppo after the death of Ridwan, their protector, they had found a new sanctuary in Damascus and Al-Mazdaghani was completely in their power.
According to the chronicler Ibn Al-Athir, faced with the hostility of the population, the Assassins planned to open the gates of Damascus to the army of King Baudouin II in exchange for the city of Tyre where they would have set up a much more safe base for their operations.
I'm basing on ''Crusades seen by the Arabs'' of Amin Maalouf who quotes Al-Athir, the chronicler writing an agreement had been done:
Al-Mazdaghani had written to the Franks to propose handing them Damascus over if they accepted to cede them in exchange the city of Tyre.The agreement was done. They had even agreed on the day, a friday.
Maalouf enlight some doubts about that claim but points that it looks not unlikely.
A few weeks after the ''conspiracy'' was discovered, al-Mazdaghani being murdered on Buri's order and the Assassins massacred, the army of Baudoin II attempted and failed to take the town.
However, later, the Assassins delivered to the Latins the fortress of Banias.
Let's imagine that the plan of Assassins worked. We can think that Zengi who had just been entrusted by Sultan Mahmud II Mossul and Aleppo would attempt to take Damascus back from the Latins, but Latins of the time could have matched him.
Then, Zengi would have to come back to Mesopotamia to fight against the new attempt of Abbasid restoration by Caliph al-Mustarshid. IOTL, Zengi barely avoided capture by the soldiers of al-Mustarshid, saved in extremis by Ayyub, governor of Tikrit, who could have merely handed him over to the Caliph.
Zengi was the first Muslim commander seen in decades effectively able to push back the Latins and put an end to the internecine warfare which plagued Near East since late 11th century, a division greatly responsible for the success of Crusaders. If ITTL, Zengi is removed as by being captured and possibly executed by Caliph al-Mustarshid, there is no more significative threat for some years (a decade or two before another able leader).
IOTL, the initial defeat of Zengi had scared the contenders for the title of Sultan enough to unite behind one candidate, Mas'ud, who after his confirmation as Sultan by al-Mustarshid sabotaged the latter's siege of Mossul and later confronted and defeated him; that would also happen, with al-Mustarshid turning on Mas'ud after Zengi's defeat.
The vacuum left by Zengi would leave Aleppo, leaves the last Arab bastion in Syria isolated.
Recap:
1129 : Damascus is handed over by the Assassins to Baudouin II.
1130/1131 : Zengi, atabeg of Mossul and Aleppo, attempts and fails to retake Damascus.
1132 : Zengi is defeated by al-Mustarshid and later captured then executed.
1133 : al-Mustarshid is defeated, captured and killed by Sultan Mas'ud who then turns on his brother Toghrul.
As Zengi's sons are too young, Aleppo is still governed by one of Zengi's lieutnants and Mossul is governed ... maybe by Ayyub who could have had it by seizing it or as a reward of al-Mustarshid ...