Kiddushin 13 - Questionable Goods
A certain man betrothed a woman with a mat of myrtle twigs. After she accepted it, those present said to him, "But the mat is not worth a prutah!" He replied, "Let her become betrothed with the four coins which are wrapped inside." The woman retained the money. Is she betrothed?
Since her silence occurred after the money has been given, it is meaningless, and she is not betrothed.
If a man betrothed a woman with goods which he grabbed or stole from her, then, if they had previously agreed to marry one another, this betrothal is valid.