Roofer removed my furnace vent and now expects me to pay him to reinstall one. What do I do?

When I bought my house 3 months ago the contract stated that the sellers would replace the roof. They did and 1 month after moving in I was in the attic and noticed the vent for the furnace was cut off and right above it there is new wood (where the vent used to go through the roof). The roofer says that the sellers asked him to remove the roof. The roofers gave a 5 year umable warranty on their workmanship. It seems to me that workmanship includes doing work that meets code and does not cause carbon monoxide to leak into the house and cause condensation to appear inside the roof (directly above what is left of the vent). My HVAC guy told me not to turn the furnace on until I get it revented. What should I do? Whose responsibility is it to fix the furnace vent? Thank you for your advice.