The money trail
Matthew Levitt, writing at Middle East Strategy at Harvard describes how tracking terror financiers strikes blows against terrorism all out of proportion to the effect of individual jailings and prosecution. "The deterrent, preventive and disruptive benefits of the financial war on terror are significant. ... Major donors inclined to finance extremist causes may think twice before putting their personal fortunes at risk." Indeed the big payoff from going after terror funders may lie in the evidence that you can't take to court.
Following such [money] trails can reveal conduits between terrorist organizations and individual cells. ... Securing convictions for financing terrorism may be difficult, and financial designations may not drain the swamp of all available terrorist funds, but they effectively deter, preempt and disrupt the activities of terrorist support networks.
One of the more interesting challenges facing financial counter-terror sleuths is whether they can penetrate the network of hawaladars, a form of money transfer common in Islamic countries. "Hawala (also known as hundi) is an informal value transfer system based on performance and honor of a huge network of money brokers which are primarily located in the Middle East, Africa and Asia." It's features seemed designed to evade financial tracking.
The unique feature of the system is that no promissory instruments are exchanged between the hawala brokers; the transaction takes place entirely on the honor system. As the system does not depend on the legal enforceability of claims, it can operate even in the absence of a legal and juridical environment. No records are produced of individual transactions; only a running tally of the amount owed one broker by the other is kept. Settlements of debts between hawala brokers can take a variety of forms, and need not take the form of direct cash transactions.
The hawala system plays a crucial role in moving money around the Pashtun diaspora, from Afghanistan to destinations abroad. It is allegedly used to move the proceeds from the opium trade. Consequently, it's not beyond the realm of possibility that it is also employed to move money among terror groups. At some point, the hawala system merges into the formal banking system, playing a role not unlike the darknet to the open Internet. It is a subterranean system which surfaces occasionally to where it can be tracked. But to follow it down into the depths requires knowing how the system works and getting among the hawaladars. Wikipedia describes its workings.
The hawala system relies on the extensive networks of mainly Pashtun diaspora in Iran, Pakistan, Saudi Arabia, Europe, and the United Arab Emirates. The groups link to the political, religious, and socio-economic networks within the core countries of Central Asia, three of which share a border with Afghanistan, and well beyond the region to the Middle East, the UK, and the US. ...
Trading families in the Pashtun Mafia ... often explained how they would establish their own hawala businesses as a "support center for their trade. ... Helmand not only accounts for more than a quarter of the total area under poppy cultivation and opium production in Afghanistan, it also dominates the country's opium trading networks. Located at the heart of the Pashtun belt, the province is therefore ideally placed to exploit the extensive Pashtun trading networks that reach many of the world's key financial sites.
From the cursory description the hawala system seems to be dependent on key nodes; it operates on an honor basis, and it's interesting to speculate how a given hawala chain would be affected by a penetration and a compromise of reputation.