The UN was paying local warlords to not hold up their convoys at gunpoint? Or the UN would pay them once held up, to help ensure some of the aid went to who it was meant for? (Sorry about needing clarification on such a random story, just genuinely curious! Wouldn't surprise me in the slightest...)
That sucks, and goes to show that the UN seems to have a vested interest in the status-quo in some areas rather than be working towards any real improvement. They say they are there to rebuild a country, assist the people, etc - yet put up with crap like that.
The warlord, in all fairness, has a pretty good business plan. Allow the UN to build a project, probably employing locals and some of how own people - while the UN also pays him not to attack it. Wait a little while, blow it up, and repeat - getting the UN to pay him each time. It sucks and it's shady, but he clearly figured out the UN would rather just plug away at it rather than eliminate/solve the problem so it could move onto the next.