I have no evidence whatsoever for the following theory, but it sounds plausible.
Looking at the pictures of Palestinian Arabs getting their $300 from Hamas' fundraising tour:
Now, Zahar filled his suitcase with $20 million from visiting Indonesia, Malaysia, Brunei, China, Pakistan, Iran and Egypt. Most of these nations are not places one would expect to be flush with US currency, especially what appears to be mint-condition currency.
However, at least China is heavily suspected of being a major counterfeiter of US currency.
What are the chances that the Palestinian Arabs are being paid with bogus banknotes?