Ok im no expert. But ill try guide you.
You need to set a trigger for the destroyed building and have it turn on a flag.
Infantry group need a waypoint with a Hold command (under advanced). Click edit on that hold command. Use the STOP CONDITION check the "IS USER FLAG" and pick the flag you used for the trigger.
Sorry man. I didnt mean for you to investigate this. I was just asking if you might knew the cause for the problem.
If i get more info ill let you know including all details.
Ok it worked. I got a cargo i could slingload, but after it was delivered i couldnt deploy the crate.
EDIT: Weird. First time it failed. Second time it worked, even with a damaged crate.