Finance structure to help them buy new house?

[George Maka]

I have a motivated seller that has a house of worth $600k and and owes about 300k on it. They're an old couple and they're hoping to sell it for 600k, take their gains and buy a house outright with no mortgage in a cheaper area up north.

Other than paying the 600k, how can I structure this where I can take ownership of their home, but still allow them to take their equity and buy a house?

Thanks,

George