Subtract TotalExpense from BudgetAmount In Budget Table for a specific date
To start with, JOIN and GROUP BY

select
    b.BudgetDate,
    sum(b.BudgetAmount) - coalesce(sum(ex.ExMount),0) as BudgetLessExpenses
from dbo.Budget b
left join dbo.Expense ex
on b.BudgetDate=ex.Date
group by b.BudgetDate


BTW. Shouldn't there be a UNIQUE constraint on [Budget].[BudgetDate] ?