It depends on how the out of range transactions have been eliminated. If either by conditional suppression or by Group Selection (Record Selection is always superseded by MAS) then the totals will still be off for Open Item customers because the group sum will include suppressed amounts and even those eliminated by the Group Selection.
However, you could use the Group Selection criteria for your date range condition and change the formula @TotalsOfInvoices to use just the #RunningBalance instead of the Sum({@TransactionAmount},{@CustomerNumber}). That worked for me.