Does anyone know if there is any equivalent or usage of the principle "פה שאסר הוא פה שהתיר" or a "מגו" in American law that could help illustrate the principle?
A basic explanation of the principle of פה שאסר is that when a person makes a claim that includes a previously unknown piece of information, the person can qualify the statement. For example, if a person were to say to his friend "this land belonged to your father, but I bought it from him" the person would be believed. Although he has no evidence that the land belongs to him now, since the son did not know that his father even had this piece of land, the person is believed because had he kept quiet the son wouldn't have even known about the land in the first place.
Another example would be if a woman, who is not known to have been ever married, says that she was married (Therefore Prohibited to other men) but is now divorced (and therefore permitted to marry another). She is allowed to marry another man without proof of divorce. The reason is the very statement that would have prohibited her also includes a statement that permits her.
In other words my understanding is that we view the two statements as a unit. If we are going to believe the first part of the statement then we should believe the second half. If we don't accept the second half, then on what basis are we accepting the first half.
This is a brief treatment of the subject. I hope it gives some clarification of what I am looking for.
Are there any examples in American Law?
For example of Halachik usage see כתובות דף טז