Being a member of ISIS pays. But does it pay well? Not anymore. Leaked documents from inside the Islamic State's territory revealed that due to wartime pressure, the terrorist group is slashing its members' salaries by 50 percent, forcing them to wage war and establish their caliphate on lower wages.

"Because of the exceptional circumstances that the Islamic State is passing through, a decision was taken to cut the salaries of the mujahedeen in half," the memo read in part, according to the Syrian Observatory for Human Rights.

"No one will be exempt from this decision no matter his position, but the distribution of food assistance will continue twice a month as usual," the statement continued.

Data from the Congressional Research Service indicates that ISIS soldiers used to earn between $400 and $1,200 a month, while U.N. researchers found that the engineers and technicians can make upwards of $1,500 in the same period, according to CNN.

The militant group had been moderately successful in the past, making most of its money by enforcing a variety of taxes on the populations in Iraq and Syria that it has conquered, while the rest is made through oil, bank looting and kidnapping. Two-thousand fourteen was particularly profitable, with ISIS raking in $2 billion.

"The Islamic State is certainly the best financially endowed terrorist organization in history. That is particularly due to its ability to govern ungoverned spaces," said Andreas Krieg, a military scholar at King's College London in Qatar. 

The group gave no explicit reason for the cutbacks, merely citing the Koran, where it says the concept of "jihad through wealth" is more important than the "jihad of soul," according to The Jerusalem Post.

However, the announcement of the financial crisis, which came in December, coincided with a series of U.S. airstrikes taking aim at ISIS's oil business, destroying oil trucks, storage tanks, mobile refineries and other oil field equipment. It's almost guaranteed that ISIS is in even worse shape in January after the U.S. destroyed a cash stockpile in Mosul.

To make up for the loss, ISIS's governor in Mosul issued a fatwa, permitting its soldiers to earn cash by taxing locals.