A mosque which was erected 100 years ago, was removed and rebuilt again by
citizens who donated their money on the basis that the structure was to be used solely as a mosque. Prayers were offered for an entire year on the ground floor after which an upper floor was built. One of those in charge of the mosque designated the ground floor as a reception hall and the upper floor as a mosque. Please bear in mind that the ground floor is used as a reception hall only as needed. In the absence of any reservations, it is refurnished as a mosque.
Is it permissible to convert the ground floor into a reception hall in this manner?

It is impermissible to use any part of the mosque which was endowed to Allah for any purpose other than worship, either on a temporary or permanent basis. It is impermissible to rent or sell any part of it or use it for any other purpose except prayers, i'tikaf [spiritual retreat], dhikr [remembrance of Allah], education and the public interest of Muslims without encroaching upon the rights of those offering prayers. Allah Almighty says: "And mosques in which the name of Allah is much mentioned." [Qur'an 22:40] And "And who are more unjust than those who prevent the name of Allah from being mentioned in His mosques and strive toward their destruction." [Qur'an 2:114] And "In mosques which Allah has ordered to be raised and that His name be mentioned therein; exalting Him within them in the morning and the evenings."
[Qur'an 24:36] And "And [He revealed] that the mosques are for Allah, so do not invoke with Allah anyone." [Qur'an 72:18]

Based on the above and in reference to the question, it is impermissible to take part of the mosque to establish a reception hall, whether temporary or permanent.