All perfect praise be to Allah the Lord of The Worlds. May His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
In principle, whatever items are placed in the mosque are a Waqf for the benefit of all Muslims and take the rulings of Waqf. According to Islamic Law, an endowment has its special rulings and conditions by which an endowed item is meant to serve only the purpose for which it was endowed in the first place. This item is the property of none and it is unlawful for any individual, no matter who he/she is, to use it for personal benefit.
Accordingly, furniture placed in the mosque to be used by all prayer performers takes the rulings of endowment in the sense that it is a public benefit. Therefore, it is unlawful to sell, gift or lend it to a private party for doing so defies the purpose for which it was endowed in the first place.
Ibn Hajar (May Allah have mercy on him) was once asked: "Is it permissible for anyone to move the water donated to the mosque for purity purposes to their house to use it for the same purpose? This is along with banning prayer performers from using that water although they need it. He said "No." and regardless of whether the water is to be used for purity purposes or not or that prayer performers are banned from using it or not because it is unlawful to move endowed water to a place other than that to which it was endowed in the first place." {Al-Fatawa al-Fiqhiyah al-Kubra, V.3:266}.
In conclusion, since it is unlawful to move the water endowed to the mosque for purity purposes to be used outside it for the same purpose, then it is unlawful to take chairs from the mosque to be used outside it for personal benefit. It is the duty of every Muslim to preserve endowed property and avoid any act of transgression against it. Moreover, whoever committed an act of this sort must make compensation for the lost item (If it exists no more) or pay money instead of using that item, along with making repentance and seeking forgiveness. And Allah the Almighty knows best.
