How does one implement "zachin leadam shelo befanav" to give a gift on Shabbat?

Question

I am aware that we don't give gifts -- or, more specifically, transfer ownership of an item -- on Shabbat or Yom Tov. I once asked a question about ransoming the afikoman and received this comment:

... That issue can be circumvented, I think, by having a third party accept the gifts on behalf of the intended recipients before Y"T. This is OK to do without their knowledge because "zachin le-adam shelo befanav" - "we can do unmitigated good for someone without their presence (or knowledge). – Isaac Moses Feb 20 '12

I understand that this is also done when shuls present a bar/bat mitzvah with a gift on Shabbat. And I have seen the "kinyan on another's behalf" approach used recently by somebody who I know to be careful in halacha.

My question is about the logistics: if one wants to effect such a transfer using an agent to accept in advance on another's behalf, what must one do? Specifically:

• Are there any restriction on who the agent can be?

• Is there any specific declaration text that either the giver or the agent needs to say, or is anything that gets the idea across ("I am giving this to Ploni"/"I accept on Ploni's behalf") ok?

• Can this be done for an unnamed person that can be identified later, such as "the child who presents me with the afikoman"?

• Can this be done for unnamed people who aren't identified in advance, such as "any guests I have for Shabbat lunch" (maybe you made up something in advance, like bentchers or something to commemorate a simcha)?

DoubleAA linked this page from Hebrew Wikipedia in chat; possibly there are answers to be found within, but I can't read it.

Answer 1

See here for a general overview and various opinions on giving gifts on Shabbos.

.. the Beis Yosef (OC 527) allows one to give a gift on Shabbos if it will be used for Mitzva purposes. This is the basis for allowing one to give their Lulav and Esrog to another as a gift on the first day (or 2) of Sukkos.

The Magen Avraham (OC 306:15) question the practice of giving presents to a Chassan who delivers a speech on Shabbos. The Chasam Sofer, (Hagaos Hachasam Sofer OC 306), however disagrees as such a gift serves a Mitzva purpose by giving true kavod to the Torah. The Aruch Hashulchan extends this , allowing one to give wedding presents at the Shabbos Sheva Brachos, as doing so brings Simcha to the Chassan and Kalla; itself a great Mitzva.

Likewise, R’ Ovadia Yosef (Yechave Daas 3:21) allows giving Bar Mitzva presents on Shabbos (especially Seforim that may be used that day) as they can serve to encourage the boy in his Torah learning and religious lifestyle.

The Shemiras Shabbos Kehilchasa (29:29) writes that one who wants to give a gift on Shabbos should give it over to him via a proxy before Shabbos. The proxy acquires it on behalf of the receiver.

As we can't trust children to properly give something back (See Kiddushin 19a; Shulchan Aruch 658:6), the proxy needs to be a adult (man or woman).

There is no specific formula, though the proxy should understand the role that they are serving.

One cannot perform this in the case of 'whoever finds the afikoman' as it is considered something that does not yet exist (Davar Shelo Ba L'Olam). Being that we typically understand that what dependent children own technically belongs to their parents, it wouldn't be an issue for their parents to give their younger children a gift on Shabbos or Yom Tov.

Likewise, it can't be done for unnamed guests, though it may be permissible as it is for mitzva use..

Answer 2

In short: I want to gift a [whatever] to my friend Yonah (gender-neutral) on Shabbat. So before Shabbat, I go to my friend Simcha (gender-neutral, but over the age of bar/bat mitzva) and say: "I would like you to acquire this on behalf of Yonah." I hand it to Simcha, and Simcha picks up the [whatever] a few inches in demonstration of acknowledgment. The [whatever] now belongs to Yonah.

This is also, by the way, how you put a "real" eruv into effect. After you put the string-and-posts around your neighborhood, converting it into one big courtyard, you need to designate some shared food that's available for all the residents. So the rabbi takes a box of matza (it's convenient and nonperishable) and hands it to -- the eruv chairman, the shul president, whoever -- "please acquire this on behalf of everyone in this area."