The only idea I have is a new mortgage program that allows someone who is selling to finance another home to keep their mortgage rate on a portion of the new mo

The only idea I have is a new mortgage program that allows someone who is selling to finance another home to keep their mortgage rate on a portion of the new mortgage. For instance, I have a 3% mortgage on a $300,000 home. I want to sell it and purchase a $700,000 home. I can sell my home for $500,000. Current mortgage rate is 7%. If I could sell my home and purchase the new one with a mortgage that is 3% on 300,000 and 7% on the additional 200,000 I think it would make selling quite a bit more enticing. This scenario is assuming I would put the $200,000 I made in equity as a down payment and I am ignoring fees/commissions for simplicity sake.