I am confused.
If all the rebels are leaving the city in one big convoy... simply bomb the convoy from the air and be done with this.
easy peasey.
I believe that they are shuttling buses in groups, and many jihadis are driving their own vehicles. Each group is about 1,000, including lots of non-combatant women and children (mostly the families, widows and orphans of the jihadis).
Pretty much all of them have cell phones, so word of a double cross would get out within the minute.
This is apparently a deal between Turkey and Russia - so if they want the credibility to do a bigger settlement later, they need to maintain their credibility.