Like the others, I find this very confusing. You agree that the thermostat wasn't correctly wired. You agree that the furnace had problems that you have had repaired. I hope that the furnace has been "tuned up" and is currently working well I presume that you have seen the actual bill for the propane delivery. To me it seems that you do not owe for the normal heating of the house, only for the large loss of gas. So, how much was for "normal heating"? It may be hard to figure this out. Perhaps you could assume that 50 gal was normal use and offer to pay for 250 lost gal. That may be around $600. So, negotiate.
Breaking the lease is a separate issue.

