Cannot compare between 2 source control versions

I have the latest version of SQL Compare Pro and am trying to compare against my latest commit (head) to the previous commit version. The goal is to create a simple deployment script to a server not able to be reached by sql compare. This seems like a pretty simple and straightforward thing to do, however when I select a previous git version for the target I get a negative number in red in the version drop down and the 'compare now' button is grayed out and deactivated. There has to be a way to do this that I'm just missing. I saw a similar question in the sql source control forum and the red gate employee suggested sql compare, but I can't see how to make it work.