Midrash su Levitico 27:31

10) (Vayikra 27:31) ("And if redeem, will redeem, a man of his ma'aser, its fifth shall he add upon it.") "redeem, will redeem": to include his wife. "and if redeem will redeem": to include his heir. "a man": to exclude a minor. I might think to include one who is nine years and one day (and above); it is, therefore, written "and if redeem will redeem, a man." "of his ma'aser": and not all of his ma'aser — to exclude (from redemption) ma'aser which was brought into Jerusalem and then taken out. (Once it was in Jerusalem, the mitzvah of eating it in Jerusalem devolved upon it, and it can no longer be redeemed), and to exclude (from redemption, ma'aser) which does not have (in itself or in the fifth) the value of a perutah. "shall he add upon it": so that it and its fifth are five (equal parts. See 2) above).