Free Membership Billing invoices (Membership 2 pro)

Hello,

I have the plugin membership 2 pro installed on my site, now i want the membership signup to be free and be able to manually create invoices for users.

When i want to do so, the billing option gets invisible and the payment button gets invisible for users too, so the payment does not work while i want the membership to be free. But it works when i select a fee for membership signups, so is there any solution for this?