Options for selling personal residence to a family member

[Mindi Marino]

We'd like to like to sell our personal residence to one of our sons and his wife. Market Value 750K, balance owed 155K @3.75% fixed. Two questions:

1) Taking back a second for anything above what they qualify for (we don't need everything up front.) Any potential issues/problems I should be aware of?

Example: They pay us 500K, we take back 250K at x% for X term

2) Our current 155K 3.75% first is not assumable. Is there any "creative" way that we could somehow pass on the benefits of that to them, and still let them qualify for the additional money we'd need to "cash out" and live on when we move? 

Any other ideas/suggestions welcome

Thank you!