How did you latch a dog with a human seat belt? They aren't designed for animals and won't hold them in a crash and could possibly cause more damage without using a proper dog seatbelt. I wouldn't do that again

To your original question: can you unbolt the bottom of the belt to allow it to recoil farther than it would normally go? That may fix it

 

He may have been using the Jeep seatbelt to hold the dog seatbelt. It sounds like it was being used to buckle the dog into the cargo area so the belt was flipped to the back of the seat.

 

I don't know what you can do with yours right now, but the belts are gravity fed. I found this out when I took them off the roll bar to spray bedliner and then I couldn't get them to relax enough to put back up. I had to hold the spring base up and let it hang loose for awhile and it let go finally.