The Torah requires a father to "redeem" his firstborn son by paying a Kohen five silver coins. The Shulhan Aruch (Yoreh De’a 305:1) rules that practically speaking, this means giving a Kohen the value that five silver coins were worth in the time of the Torah. In the currency used in the times of the Shulhan Aruch, he writes, this amounts to 30 Darham of silver. A Darham is 30 grams, and so the amount of pure silver that must be paid to a Kohen to fulfill the Misva of Pidyon Ha’ben is 90 grams. The custom in Jerusalem was to add a Darham to ensure that the Misva is properly fulfilled, thus bringing the total to 93 grams.
It must be emphasized that one does not have to give the Kohen five coins. As long as one gives the Kohen the required amount of silver, he has fulfilled his obligation, even if he gives a single coin containing 93 grams of silver, or even a Kiddush cup or some other object that contains this amount of silver.
Moreover, one may give the Kohen any object of value that is worth 93 grams of silver. The important exceptions to this rule are real estate, and a Shetar (a document). One cannot transfer a piece of real property to a Kohen for the Misva of Pidyon Ha’ben, even if it is worth the same as 93 grams of silver, and one cannot give a Kohen a document – such as an IOU – that entitles him to collect this value. Since the Shetar is just a piece of paper, and does not have intrinsic value, it cannot be used for the Misva, even though it can be used to claim money in the amount that is required.
For this reason, one cannot fulfill the Misva of Pidyon Ha’ben by giving the Kohen a check for the required amount. A check has no intrinsic value, and is simply a document instructing the bank to transfer the stated amount. As such, it cannot be used for fulfilling the Pidyon Ha’ben obligation.
The more difficult question regards the use of regular money – such as dollar bills – for this Misva. On the one hand, one might argue that a dollar bill is no different from a Shetar. After all, the bill is nothing but a piece of paper that entitles a person to receive goods, just like an IOU, and thus it should perhaps be invalid for use for Pidyon Ha’ben. On the other hand, one could contend that once the government establishes this paper as legal tender, and it is legally and conventionally treated as an object with intrinsic value, it should be no different than other objects of value, which may be used for Pidyon Ha’ben. This question is addressed by the Hatam Sofer (Rav Moshe Sofer of Pressburg, 1762-1839), in one of his responsa (cited in Pit’heh Teshuba, 305:7), and he leaves this issue unresolved.
Due to this uncertainty, one should not use regular money for the Misva of Pidyon Ha’ben, and should instead use silver coins. Although it is common to use five silver coins to fulfill this Misva, this is not strictly required, as mentioned earlier. As long as one gives the Kohen a total of 93 grams of silver, he has fulfilled this Misva. One who uses five silver coins must ensure that each coin contains at least 19 grams of silver, so that the total will reach 93 grams.
Rabbis who officiate at Pidyon Ha’ben ceremonies must familiarize themselves with these Halachot and ensure that the proper amount of silver is given to the Kohen in fulfillment of this Torah obligation.
Summary: One may not use a check for the Misva of Pidyon Ha’ben, and it is uncertain whether the Misva can be fulfilled with regular paper money. Therefore, one should use silver coins for this Misva, and he must give the Kohen a total of 93 grams of silver. It is customary to use five silver coins, but any number of coins may be used, as long as the required amount of 93 grams of silver is given to the Kohen.