I ran into the same thing with Oklahoma Natural Gas about 10yrs ago, when I bought my former house out of foreclosure. Even though I had a sales contract and all the paperwork involved in the sale, they wouldn't turn on the service as there was a balance owed by the previous owner.
It might have been different if I was just transferring service, but I had lived out in the country and had rural propane service before.
I ended up paying it along with the deposit for a new account, as I had no choice if I wanted heat in November, then raising hell with the State Corporation Commission and getting the local news involved. It took a month, but they finally refunded my account for the previous owners balance and my deposit.