Bechorot 51 - Firstborn Redemption with Real Estate
To redeem a firstborn one can give to a Kohen either five shekel coins or goods worth that amount. However, lands, slaves, and loan documents cannot be used for redemption. Why not? When the Torah said, "A firstborn is redeemed from one month old", it included all means of payment. When it continued, "According to the valuation of five shekels of silver", it limited the available means somewhat, and when it again generalized, "You shall redeem", we interpret it to mean that only payments similar to the specified ones are included. Just as coins are movable and have intrinsic value, so too all similar items are included. What's excluded? Lands, because they are not movable. Slaves, because they have the laws of the land. Loan documents, because they are pieces of paper not worth anything by themselves, but only because of the obligation recorded in them.
If the father wrote the Kohen a note which said "I owe you five shekels", he has created the obligation and needs to pay at. However, this does not redeem his son, so he needs to pay another five shekels. If the Kohen wants, he can return five shekels as a present.
Art: Jan Steen- A woman counting coins at a table
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