Ma’aser Sheini 4:3

If the owner of the produce wants to redeem it for a sela (a denomination of currency) and another person also offers to redeem it for a sela, the owner takes precedence because he adds one-fifth. If the owner offers a sela and another person offers a sela plus an issar, the other person takes precedence (even though his offer is less than the owner’s once the extra fifth is added) because he is offering more in the principal. When one redeems his own second tithe, he must add one-fifth to the value whether he grew the produce himself or received it as a gift.

Ma’aser Sheini 4:4

One may employ a “workaround” to avoid paying the extra fifth, by giving the principal to his adult son or daughter, or to a Jewish servant, and having them redeem it for themselves. One may not do this with a minor child or a non-Jewish servant because when these people act as his agent, it is considered doing it himself (and the fifth must be added).