Where did Sheikh Abi-Ishaq al-Huwayni get these ideas? Why, from Muhammad, the prophet of Islam, of course. Umar, the second caliph, said: "I advise you to fulfill Allah's Convention (made with the Dhimmis) as it is the convention of your Prophet and the source of the livelihood of your dependents (i.e. the taxes from the Dhimmis.)" That is, subjugating the dhimmis and bleeding them dry is to be the source of the Muslims' livelihood.
"Egyptian Shaykh: Jihad Is Solution to Muslims' Financial Problems," from Translating Jihad, May 31:
In this short audio file, prominent Egyptian Salafi Shaykh Abi-Ishaq al-Huwayni explains that Muslims' financial difficulties are due to the fact that they have abandoned jihad. [...]
Transcript:We are in the era of jihad. The era of jihad has come over us, and jihad in the path of Allah is a pleasure. It is a real pleasure. The companions (of the Prophet) used to compete to (perform jihad). The poverty that we’re in—is it not due to our abandonment of jihad? But if we could conduct one, two, or three jihadist operations every year, many people throughout the earth would become Muslims. And whoever rejected this da’wa, or stood in our way, we would fight against him and take him prisoner, and confiscate his wealth, his children, and his women—all of this means money. [...]