No advice, but I feel your pain. I was trying to sell during the crash in 2008, having to live with my STBEX, and it was hell. It took 3 years to sell. Meanwhile, I moved out, so he had no motivation to sell, as he was sitting pretty.
He would leave the house in a mess, so I would have to tidy up his crap, and clean and garden, before every viewing.
He moved a woman in, and I would have to pick her underwear up off the bedroom floor. I remember one time, he had let the garden get so overgrown with weeds, and someone was booked in to view it. I was on my hands and knees in the mud, pulling out weeds, in torrential rain, it was thundering and lightening, and he came back with his GF, and stood over me, yelling at how I had ruined his day by being there. Truly hideous times.
But nothing lasts forever Op. Hopefully you will be out of there soon. I absolutely wouldn't give him a penny more than he is due - you'd regret it, I think.