Law term regarding Subject to deals

[Jim Sharp]

I'm trying to recall a law term... in relation to the following situation.

You buy a house sub2, you disclose to a lender you are the new owner. Then a year down the road they decide to invoke the due on sale clause.

The law pertains to not being able to have knowledge of something, then wait until it is in your benefit to invoke the knowledge.

It's not all that important... just driving me crazy that I can't remember it.