Bava Batra 40 - Secret Gift
A gift document written in secret cannot be used to collect the gift. The giver may have already given the property to someone else, and is now hiding from the first recipient, and then the gift is not his to give. Or, he may write this gift document to fool all later recipients of this property. Because of these suspicions, a secret gift document is considered void.
One woman said to a man, "I will marry you only if you give me all your property." His son wanted a share. The man then told witnesses, "First hide yourself and write a gift of all my property to my son." Both gifts were annulled: to the woman - because it was given under duress, to the son - because it was only for subterfuge.