There's a BalanceRemaining
field which gives you what you're looking for.
...
<AppliedAmount >AMTTYPE</AppliedAmount>
<BalanceRemaining >AMTTYPE</BalanceRemaining>
...
Code should look something like this:
if (InvoiceRet.BalanceRemaining != null)
{
double BalanceRemaining71 = (double)InvoiceRet.BalanceRemaining.GetValue();
}