I'm assuming  you have a Cash or Cash on Hand Account that you keep track of your cash.

If you use Bill Pay, simple add the invoice in as you normally would and then go to pay the invoice.  The Pay Bill screen will default to your Checking Acct, but just use the drop down menu and choose the Cash Acct.

If you do not want to use Bill Pay, you can simple go into your Cash Account Register and enter the Payee and the correct Expense Account.  Either way, you're paying the invoice with Cash.