How to get Pro Sites to unforce SSL after payment

I have a multsite install running Pro Sites. Its setup for subdomains so new sites have URLs like newsite1.domain.com, newsite2.domain.com, etc.

We are using Stripe to take payments on site so Pro Sites is set to force SSL for checkout.

The SSL certificate we have is only for the main domain.

The problem is that after payment is completed on the main domain, the user is taken back to their main install on their subdomain.

Since the SSL certificate is only for the main domain, the browser gives a warning saying "This Connection is Untrusted" because to secure connection is still active.

How can I get Pro Sites to unforce SSL and return to regular http:// so there is no browser warning once payment is made?