Calculate due amount over available balance against customer ID in Vb .net

I have created web application which will add the values into database As customer id, available balance, due Amount and have to calculate the due amount against available Amount from client using vb .net.

For Ex. - I have three Bills in the Database as,

customer id | Due Amount

APP1 | 5|

APP1 | 5|

APP2 | 10|

Now from client APP1 , i have received 7 Total, so i have to clear the due amount according to Total Amount Received. That means, i have to update the due Amount, So APP1 will clear, have to update due amount to 3, For APP2, 10 deduct so due amount will be zero .

So Please help me to write this logic in vb .net.

did you try it out first on a piece of Paper
or in a Excel sheet ?

what fields would you need?
I could think of...
CustomerID
DateDue
StartBalance
AmountIn
PaidDate
PaidAmount
Balance
DateBalance
OverFee
OverFeeAmount

Exactly the calculation of paid and overdue amount will be based on customer ID , so if the customer has available balance it should be calculated and deducted , the issue here that the same customer may have more than one record that related to previous months so i need this be calculated separately based on month