I do use Sona (in theory – at least I help colleagues use it). You certainly should be able to use the link that you would send from Qualtrics to Sona as the redirect URL in Pavlovia instead,
As far as I remember, participants have to click OK after they leave the experiment to save the data and the redirect follows that.
Since you said var=1 in your example, I assumed you had an embedded variable called var. So your URL actually contains fin=1?
When they come back to Qualtrics they are going to be at the beginning again, not at the point they left (I think).