Is placing money in the mosque's building fund considered charity (sadaqah), and does it carry a greater reward than giving charity to the poor? And is giving charity to cancer patients considered ongoing charity (sadaqah jariyah)?

The money paid for the construction/renovation of a mosque is a charity for which its doer will be rewarded. The priority between the construction/renovation of a mosque and giving charity to the poor depends on the need. If the need of the poor is urgent and the mosque is not in dire need, then giving charity to the poor is more优先. However, if the mosque is in need and the poor are not in dire need, then the construction/renovation of the mosque takes priority. Giving charity to poor cancer patients is among the best deeds, as the Prophet, peace and blessings be upon him, said: "Whoever relieves a believer of a worldly distress, Allah will relieve him of a distress on the Day of Resurrection." Enduring charity is an endowment whose benefit continues.