Is it permissible to agree on a future price for a service ($25 per square meter) and then adjust it to a lower cash price ($21 per square meter) after the work is completed and before payment?

"Leasing (Ijarah) is similar to selling in its rulings. This transaction falls under the issue of 'reduce and hasten,' which is the condition of reducing a debt in exchange for hastening its repayment before its due date. This issue is not permissible according to the majority of scholars. However, it is permissible according to Ibn Abbas (may Allah be pleased with them both), and Ahmad held this view in a narration chosen by Sheikh al-Islam Ibn Taymiyyah and Ibn al-Qayyim. The Islamic Fiqh Council has adopted this opinion, stating in its resolution that reducing a deferred debt in exchange for hastening its repayment, whether at the request of the creditor or the debtor, is Islamically permissible and does not fall under prohibited usury if it is not based on a prior agreement. Based on this, there is no blame on the questioner for what he did."