Enter Find Mode 
Set Field [Table::transactionType ; "Payment" ]
Set Field [Table::TransactionType ; "Invoice" ]
Constrain Found Set
For more exampls of scipted finds, see: Scripted Find Examples
I have another question concerning this same report. I have a subsummary based on the transactionType field. So, it summarizes the transactionType "Payment" and the transactionType "Invoice." The report displays properly. Now, I would like to be able to subtract the subsummary value for "Invoices" from the subsummary value for "Payment". First, I created a getSummaryPayment and a getSummaryInvoice fields. They also display the correct amounts. However, I am not sure what to do next. The calculation "getSummaryPayment" minus "getSummaryInvoice" does not work when placed in the trailing Grand Summary, nor in the subSummary parts. Perhaps what I want to do is not possible?
How did you define these two new fields?
are they calculations with the getsummary function or something else?
The following should work:
Define a pair of calculation fields that will be blank unless the transaction is of the correct type:
cPaymentAmounts: If ( TransactionType = "Payment" ; amount )
cInvoiceAmounts: If ( TransactionType = "Invoice" ; amount )
Then a pair of summary fields can compute the total invoices and the total payments and then you can define yet another calculation field to subtract the two.
That worked. Thank you again!