Calculate to dependent column in SQL Server

I wrote sql as below, it's correct answer:

select id, customer, month, isnull((select top 1 (isnull(money, 0)-isnull(paid, 0)) from salary where id=mysalary.id and CONVERT(date, '01/' + month, 103) < CONVERT(date, '01/' + mysalary.month, 103) order by id Desc),0) from salary mysalary)

thanks for your help