Process Invoice Using a Cash Sale

In Transaction > Sales > Create Sales Order > New, when billing a sales order it leads to a cash sale invoice. 

Factors that affect Billing Sales Order are Terms and Payment Method. Payment Method takes precedent over terms and a Cash Sale will be generated instead of Invoice.

To confirm:
1. Create a copy of a Sales Order with payment method selected in Billing tab.
2. Remove the payment method in the Billing tab.

3. Click Save.
4. Click Bill.

Note: It should now lead you to an Invoice form.