Calculating a sum of records in a field

[alex243]

I am working on a form called Invoices with a subform TransactionsForInvoice. It shall show all transactions that refer to a particular invoice and then show a sum of the amounts of these transactions. Later I compare this sum to the invoice amount and state whether the invoice is payed or not.

The subform is based on a query that returns all the transactions for an invoice. I tried to calculate the sum of them with the Totals function or with an expression but got only the sum of all banking transactions (for all invoices).
Is it possible to calculate the sum of certain fields like in Excel? ...
Du not know

 

[stepone]

In the Footer of the subform, add an unbound textbox and set its data source to be "=sum([<fieldname>])" where <fieldname> is the name of field you wish to subtotal.