I am the Imaam of a Musjid.

Answer

Understand well that being employed does not absolve you of your duty to Allah Ta‟ala. Your first allegiance is to Allah Ta‟ala. It is haraam to obey any person, even parents, if the instruction is in conflict with the Shariah of Allah Ta‟ala. Your first obligation is to seek the Pleasure of Allah Ta‟ala, and this is possible only by obeying Him.

It is not permissible for you to perform such a Nikah which will be followed by a haraam wedding reception. Tell the people without any ambiguity that they should perform their nikah also in the haraam hall where they will be having their haraam reception. Do not perform such a nikah. Also remember that Allah Ta‟ala is your Raaziq, not your employers. If your employer (the Musjid committee) objects, resign and seek halaal work elsewhere.