Is it permissible to change the price and date of a Tajweed course after the initial agreement, even though "believers are bound by their conditions"?

There is no harm in taking a fee for teaching Tajweed (rules of Quranic recitation). It is permissible according to the saying of the Prophet, peace and blessings be upon him: "Indeed, the most deserving of what you take a wage for is the Book of Allah." The fee can be paid in advance, in installments, or deferred, according to the agreement. However, if a date for the course was agreed upon and the change of date came from those organizing it, then it is not permissible to increase the fee, because the contract has been concluded, and the delay was from their side, and Muslims are bound by their conditions.