How can the buyer complete the sale after they submit form without paying?

How can the buyer complete the sale after they submit form without paying? Can we send form back to them to complete?

You may consider using our Purchaser Order tool: https://www.jotform.com/help/460-How-to-Create-Payment-Form-with-Purchase-Order. The PO tool allows you to configure product items without requiring users to pay online. It is just a tool for manual payments. 

You already have Paypal on the form. To use Purchase Order, you will need to delete Paypal. You do not need to add the items again when deleting/adding new payment tool because the form builder will automatically add the previous items for you. 

Your second question is, however, not possible to do with the Purchase Order tool. You will need to ask your customers to Pay you manually and then you'll have to see the data in the form to verify the submissions and payments. 

Let us know if you need further assistance.