Accepting An Unintended Worldly Benefit For Worship
Even if it is one session, one can still take payment for their time. The point is that it can only become objectionable when there is some ethical problem in such arrangement. For example, asking for a payment afterward when initially it was being taught gratuitously. Another example would be to exploit people in making payments because of shortage of teachers or despite people being financially poor to afford such payments. These are some ethical issues which will make receipt of such payments objectionable in some instances and prohibited in some. It is academically more sound approach to assume that prophet Muhammad (sws) also saw some ethical problems in such transactions which made him dislike taking of such payments, because in order to declare something prohibited, we need explicit statement to this effect from Quran and Sunnah.