I’ll be blunt:
You failed to advise them that the house had no source of heat, misidentified the type of heating system installed, and failed to report the gutted gas furnace.
Now you need to pay the cost to cure this defect, or pay whatever your contract states. The buyer may not accept twice the fee refunded, but, who knows.
I imagine this rooftop gas pack should have had visible signs of “something’s not right” and a quick look inside the unit would have revealed the cobbled-together A/C system.
We all make mistakes, this will likely be the very last time you make this type of error.