Then enable payments normally. Go to Settings → Payments, choose the processor(s) that you will use, provide your data and assign prices. Tick the box Enable payments for this form.
Finally, go to Settings → Rules → Form Rules and establish the following rule: If pay offline is checked then redirect to website http://www.yourpage.com/, where http://www.yourpage.com/ is the URL of the page where you want to redirect the users who chose to make the payment offline.
With these settings, users who choose to pay online will be redirected to the online payment processor. Users who don’t will be redirected to a page of your choice and will make the payment offline, in any way you prefer.