1. You can smuggle oil to companies that need them through Turkey or Sea ports controlled by ISIS.
2. Saudi Arabia refused to change the price of Oil because it was relatively high compared to the ISIS oil prices.
3. The US maybe can buy oil for cheap but they can't sell their own oil because clients prefer cheaper oil. which will influence the US income and oil companies market.
And prepare against them whatever you are able of power and of steeds of war by which you may terrify the enemy of Allah and your enemy and others besides them whom you do not know [but] whom Allah knows. And whatever you spend in the cause of Allah will be fully repaid to you, and you will not be wronged.