Is paying half the price for "mu'assel" instead of its full price—due to poor service and not changing the coals—considered a wrong or forbidden act, or is it permissible?

You are sinful and disobedient to Allah by smoking shisha and paying its price, because it is not permissible to spend money on a forbidden act of disobedience. You must repent to Allah by quitting shisha, regretting your action, and resolving not to return to it. You are not allowed to pay the remaining rental fee for using shisha, because its use is forbidden, and money should not be paid for a forbidden benefit. You must donate the remaining amount in charity.