Is it permissible for preachers to stipulate a specific sum of money in exchange for delivering lessons and lectures?

It is permissible for a da'i (caller to Islam) to receive a reward or salary for calling people to Allah, especially if he is dedicating his full time to this work. It is also permissible for him to set the fee himself or have it set by someone else. The Companions received a wage for performing ruqyah (exorcism/healing) with the Qur'an, and the Prophet, peace and blessings be upon him, affirmed that. However, it is preferable for the da'i to forgo the wage if he is wealthy and does not need it.