Taking away tax from first payment only

[shindb81]

Hi. I have a database for a small clinic.
Patients make multiple payments with various payment methods, and I have a tblPayment associated with tblAppointment.
tblPayment includes fields AmountCharged and Tax


I made a query to show total payments and to also show net income, by subtracting tax from the payment.
Say an appointment resulted in AmountCharged of $100 and Tax was $10.

My query is subtracting tax from every payment made as below:
payment 1 = $60 tax = $10 net income = $50
payment 2 = $50 tax = $10 net income = $40

I want the query to somehow subtract tax from just the first payment record for any given appointment, and not tax them twice. Can this be done?
Thanks.

[June7]

Not easily.

Tax should be accounted for in a table that records patient charges on account. Should not be any need to break up the payment, just apply payment to patient account.

[shindb81]

I see.
Thank you.