Aggregate in SQL SERVER

Question

I have the query below: 

 

SELECT sum(c.ConvertedNetPremium + c.ConvertedVAT + c.ConvertedRegistryFee) as Due
    from dbo.CashierSalesDetails c

select sum(a.AmountPaid) as payment
    from dbo.tblAgentPaymentMonitoring a

 My requirement is that, I should be able to deduct the "payment" from the "Due". below is the actual data of both tables

 

 

 

 

 I tried this code but the answer is wrong:

SELECT a.TaxInvoiceNumber, sum(a.AmountPaid) as payment, sum(c.ConvertedNetPremium + c.ConvertedVAT + c.ConvertedRegistryFee) as Due,

        sum(a.AmountPaid) - sum(c.ConvertedNetPremium + c.ConvertedVAT + c.ConvertedRegistryFee) as BAlance

FROM     dbo.tblAgentPaymentMonitoring a INNER JOIN
                  dbo.CashierSalesDetails c ON a.TaxInvoiceNumber = c.TaxInvoiceNumber
GROUP BY a.TaxInvoiceNumber