Hi Sherri
Enter the cash but leave it unapplied - unless you know in advance the invoice numbers you will be applying it to. If you know invoice numbers, you can apply the cash to the invoices even though they don't appear on the list by clicking on a blank row below the last detail and entering the invoice number manually.
To match unapplied cash against open invoices, enter a $0 check - it's easiest if you start by selecting the unapplied cash, and after that the invoices. Once the total unapplied amount is zero again, save the transaction and post it.
Steve