Here, the Mishnah is going to talk about the currency used for the Mitzvah (commandment) of Pidyon HaBen. The Torah tells us that five shekalim (shekels) have to be used in the Shekel HaKodesh (holy shekel). A shekel in the times of the Torah is equal to a selah (currency unit) in the times of the Mishnah. So we always refer to a selah, as we'll elaborate on in this Mishnah. We'll describe what type of currency we use. The five Sela'im (currency unit) used for the Mitzvah of Pidyon HaBen are paid in the currency of Tyre, of Tzor (Tyre).
There are two types of currency that we use in the times of the Mishnah. Number one is Kesef Tzori, the coins that were issued by the kingdom of Tyre, pure silver. Number two is Kesef Medinah, which was the local currency: one part silver and seven parts copper. For Pidyon HaBen and other types of de'oraita (Torah-level) payments, the maneh Tzori (Tyrian maneh) was used. If a person's ox had gored three times previously and then killed a Kena'ani (Canaanite) slave, the owner of the ox has to pay a 30 shekel fine to the owner. That fine has to be paid in the shekel of Tyre, of Tzor.
There is a 50 shekel payment for Ones (against her will) and a 50 shekel payment for Mefateh. If a person had relations with a Na'ara, an unmarried virgin in between 12 and 12 and a half years old, and it was against her will, he has to pay a fine of 50 shekel as well. If he seduced her, that means he convinced her to have relations willingly; he also has to pay a 50 shekel fine or marry her. These payments are also made in the currency of Tyre.
The 100 silver fine is paid for someone who slandered his wife—his fiancée, as we'll call her, the woman he's engaged to halakhically. There are two parts to a marriage ceremony: the nisu'in (marital union) and the kiddushin. The kiddushin goes first. A man is halakhically engaged to a woman before he goes to live with her in the nisu'in. He accuses her of having relations with another man during that time. If he did so falsely, then he would have to pay a 100 silver fine to her or her family. All of these payments are made in the holy shekel. We call it holy because it's used in the performance of mitzvot, which are holy. These payments have to be in the currency of Tyre.
You are allowed to redeem all items that require redemption, like the bechor (firstborn) or an item that was consecrated to the Beit HaMikdash, with kesef, with money itself, or with things worth money. For example, if a person needs to do Pidyon HaBen, to redeem his son, then he could do it with the five shekel kesef, the five Sela'im, or with an item that is worth five Sela'im. The exception to the rule is the Machatzit HaShekel (half a shekel) that people were giving on an annual basis to the Beit HaMikdash. The Machatzit HaShekel has to be given as coins. It cannot be redeemed for an object worth half a shekel.