Shaykh Ibn Uthaymeen was asked:
Question: Speaking in the masjid in regards to the worldly affairs, is it a sin or not?
Answer: Speaking inside the masjid is divided into 2 categories:
The first category is to cause disturbance for the people praying, reading and studying [students]. This is not allowed and it is not upon anyone to do an action that will cause disturbance for those praying, reading and the students.
The second category is to not cause a disturbance to anyone by your speech. This is if your dialogue is upon good matters then it is good.
But if [your speech] is about worldly matters, it is either from that which is impermissible or that which is permissible.
From that which is impermissible is to talk about buying, selling and renting. It is not permissible for a person to sell or buy inside the masjid or to seek rent/lease or to employ. Likewise to look for lost items, as the prophet of Allah sallallahu alaihi wasallam said
“If anyone hears a man inquiring in the mosque about something he has lost, he should say: “May Allah not restore it to you” for mosques are not built for this purpose (Hadith of Abu Huraira found in Muslim).
From that which is permissible for people to discuss in relation to the worldly matters is a truthful/honest discourse that has in it nothing that is prohibited.
[Fatawa Nur Ala Darb The Book of Prayer Volume 5 Page 622]