Giving lessons after prayers is something that is permissible, and it comes under the heading of means that are permissible in principle. This is proven on the basis of the Sunnah and the practice on a large scale that has been ongoing since the best of generations. There is nothing wrong with giving a lesson before Jumu`ah so long as it does not distract the worshippers or take up too much space, especially if there is a need for that.

It is permissible to pay fees for permissible services that all the people [in the building] pay for, even if some of them use them for unlawful purposes.

It is permissible for someone other than the one who acquired it to benefit from wealth that is unlawful because of the way in which it was acquired, but if he is able to look after himself and refrain from taking it, that is better.