Kiddushin 12 - Betrothal with Correct Value
According to Beit Hillel, the minimum amount of money needed for Kiddushin is a small coin called a prutah. This does not mean the smallest coin in circulation. Rather, it is the value of one-half of a barleycorn of silver, and it is estimated anew each generation.
If a man betrothed a woman by handing her a date with a value of less than a prutah, there is still a possibility that in some place this date is worth more than a prutah. Therefore, he should repeat the Kiddushin with a definite value, or give her a Get.