What is the legal ruling on someone who sets a material price for books or services by specifying a number of prayers upon the Prophet (peace and blessings be upon him), such as "The price of the copy is one thousand prayers upon the Prophet" or "Fifty prayers upon the Prophet" for a particular service?

It is permissible to distribute books on condition that the beneficiary supplicates for the Prophet, peace and blessings be upon him, a certain number of times, provided that what is in the books is good and beneficial. And whoever takes a book under this condition must fulfill it, for the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions, except for a condition that forbids what is lawful or permits what is unlawful." This condition does not forbid what is lawful; rather, it stipulates doing a good deed, just as stipulating prayers upon the Prophet, peace and blessings be upon him, in exchange for a benefit such as being transported to a place, or given water, and so on.