How is zakat to be paid on a sum of money comprising salaries, loan installments, and business profits, from which zakat was only paid once, and what is the ruling on paying this zakat to mosques?

Money must be withdrawn from interest-based banks, and the interest disposed of by spending it on Muslim welfare. Zakat is not obligatory unless the money reaches the nisab and a hawl (full lunar year) passes. Business profits are added to the principal and zakat is paid on the total amount. As for money acquired from non-business sources, such as salaries, zakat is not obligatory on it until a hawl has passed. It is permissible to pay its zakat in advance along with the principal. One must repent for delaying past zakat and make it up. Zakat is calculated by taking one-quarter of one-tenth (2.5%) of the money on which a hawl has passed. The deduction of debt from zakatable wealth is a matter of scholarly disagreement. Zakat may not be spent on building mosques.