Bava Batra 154 - One Who Forbids Something Can Also Allow It
The rule "The mouth that forbids it is the mouth that allows it" can be illustrated as follows. In a case where a woman who told the court that she was married, but added that she divorced or that her husband died, her first statement makes it forbidden to her to marry other men, but the second one makes it allowed. If we believe the first statement, we should also believe the second one.
When one who says, " (1) I did bequest you this gift, (2) but I was sick then, and now that I recovered that gift is invalid" we should follow the "mouth" rule and believe him. However Rabbi Meir considers the gift document as valid without certification. If so, we know (1) from the document itself, and do not believe the man on (2).
Art: Richard Caton Woodville - The Sailors Wedding