Back in 2010 when I was still in school by father co-signed with me when I was buying my house because I had very little credit history. I still live in the home currently.

Fast forward to today. He just retired last month and sold his dental practice but still owns the building. On January 1, 2020 he is going to be selling the building as well to the new dentist. In order to not pay the capital gains tax, he has to reinvest the money somewhere. He has been toying with the idea of buying my house from me at current value. I would keep the profit from the sale and then I would pay him rent on the house which would be much less than my current mortgage because there would be no interest involved. All in all it seems like a win-win for everyone.

Is this legal to do? And would this work since he is a co-signed on my loan?