I'm not sure but IRS liens take precedence over other liens. I think in order to sell the property, the lien would have to be paid off in full. This would be done out of the proceeds of the sale. If the co-owner's half of the proceeds only comes to $66k, the rest would probably be coming out of your half, so the other seller would owe you that amount.

You don't mention if the home is under contract, but if it is, check with your title company. They can give you better advice. Their lawyer may also be able to draw up a promissory note for the other seller to sign if the scenario I mentioned above is what will happen.

I think your idea of a tax attorney to deal with the IRS is a good one. He will probably also be able to advise you on what is necesssary for the sale to take place and/or draw up the promissory note if necessary.