Hi @John Chin great question
So as it turns out, there's really nothing you can do to avoid them calling in the loan. Aside from telling the seller to keep their mouth shut, and not tell the bank what you are up to, its really up to the lender to decide on whether or not to call in the loan once they've learned that the deed has exchanged hands while the loan is still in someone else's name. The lender holds the right to exercise this option, and there isn't anything you can do to stop it.
The lenders however, tend to look the other way on these deals. Foreclosing on a house means that they have to spend large amounts of money in legal, and processing fees, and since the lenders are in the money making business, they tend to ignore that these deals are happening... so long as they are still getting paid.
If you owned a store, and Person A wanted to buy a candy bar from you, but Person B behind them handed you the cash to buy the candy bar for them, would you then pay almost twice as much to keep Person B from making Person A's payment? Certainly not! That's just not good business, and that's the way the lenders tend to look at these deals. As long as the payments are being made, the most lenders wont care who is writing the check.
Now the "due on sale clause" helps not only protect you, but also lets the seller know exactly the risk involved in signing this deal. Its really at the risk of the seller, and his name being reported on credit.
Hope this helps!