Why should a single date be worth a perutah in Modai?
On Daf 12a the gemora brings a teaching of Shmuel, that if someone does kiddushin with a single date, even if the current price is a kor of dates for a dinar (and therefore the value of a single date is far less than a perutah, the minimum required to effect kiddushin), even so she is betrothed in marriage, because we are concerned that maybe a single date is worth a perutah in Modai.
We can understand this teaching of Shmuel with the gemora in Bava Kamma 103a, which says that if someone steals from another an article worth a perutah and swears that he did not steal from him but later confesses, he must pursue him in order to return what he stole, even to Modai. Why does the gemora specify Modai? It cannot be because Modai is far away, because then it should simply have said that he must pursue him until the end of the world!
But we can explain it according to the posuk in Yeshayohu (13,17) “Behold, I will arouse against you Modai, who give no importance to silver, and do not desire gold”. Thus the gemora means that even though in Modai a perutah is practically worthless because they have an abundance of silver and gold, he must still return the object to him there.
And thus Shmuel said that she is betrothed because perhaps a date is worth a perutah in Modai - because money is worth so little there it is possible that a single date is worth a perutah in Modai.