Paying for the four species before the holiday:[1]

Some Poskim[2] rule that one is required to pay for the four species before the Sukkos holiday, in order for the four species to be considered completely his from a Biblical perspective.[3] [If one cannot afford to pay the full sum prior to the holiday then he is to pay at least some of the sum as a down payment prior to the Holiday, even one Perutah.[4] Regarding one who pays by check, see footnote.[5] One who pays by credit card is considered to have made a valid payment according to all.] Other Poskim[6], however, negate the above opinion and rule that the ownership is valid even Biblically when purchasing on credit.[7] Practically, the worldly custom is not to be particular in this matter to pay before Sukkos[8], although some Poskim[9] conclude that it is proper to suspect for the former opinion.

The Chabad custom:[10] The Chabad custom is not to be particular to pay for the four species before Sukkos. [Accordingly, one may purchase the Daled Minim on credit, and pay after Sukkos, or give a postdated check to the seller. Nonetheless, according to some records[11], and as can be seen in some videos[12], the Rebbe was particular to pay the seller of the Daled Minim immediately after receiving them from the seller. The Rebbe can be seen paying with cash.]

If one refuses to pay:[13] If one purchased the Daled Minim on credit and refuses to pay the seller upon request of payment, then it is possible the Daled Minim are considered stolen, and is hence invalid.  

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[1] See Sdei Chemed Asifas Dinim Daled Minim 3:2 [Kehos Volume 4 p. 685b]; Kelalim Lamed 141:24; Kaf Hachaim 658:14; Ketzei Hamateh 624:18; Piskeiy Teshuvos 658:6 and 12; Nitei Gvariel Daled Minim 46 footnote 3; See Toras Menachem 5710 p. 85; Otzer Minhagei Chabad 14:2

[2] Machaneh Ephraim Kinyan 2, brought in Birkeiy Yosef 658:4, Shaareiy Teshuvah 658:5 [who concludes that one who is meticulous is to be careful in this matter]; Mateh Efraim 625:17 “One must beware upon buying the four species, that if they are purchased on credit, then he is obligated to pay for it before the holiday, even if he already brought it into his home”; Alef Lamateh 658:28; M”B 658:10 in name of Poskim ibid, and concludes to initially be stringent; Biur Halacha 11:6 “Mitzemer” regarding Tzitzis

[3] The reason: As there is an obligation on the first day of Sukkos for one to own the four species, and according to Biblical law only cash payment  has ability to acquire an object, as opposed to mere picking it up [i.e. Meshicha] which only acquires it from a Rabbinical perspective. [Machaneh Efraim ibid; Shaareiy Teshuvah ibid; See Rambam Mechira 3]   

Kinyan Chatzer: Some Poskim suggest that one can Biblically acquire a sale item using Kinyan Chatzer, even if he does not pay for it right away. Accordingly, if one brings the four species into his home, it is considered Biblically his even if he purchased it on credit. [M”B ibid; Sdei Chemed in name of Pesach Hadvir] Other Poskim, however, rule that Kinyan Chatzer is not Biblically valid if payment as yet to be given. [See Mateh Efraim ibid; Alef Lamagen 628:20; Piskeiy Teshuvos 658:6]

[4] Or Sameich Lulav 8:10; Piskeiy Teshuvos 658:6

[5] Regarding if a check can acquire Biblically: See Kinyan Torah 3:93 who is in question regarding a postdated check. However, a check whose date is prior to Sukkos, is considered like cash. [See Sefer Kashrus Daled Minim p. 211; Piskeiy Teshuvos 658:6; Pischeiy Choshen Halvaa 10 footnote 21; Shevet Halevi 7:222; Minchas Yitzchak 5:120; Igros Moshe C.M. 2:15; Lehoros Nasan 8:106; Ohola Shel Torah 1:84; Mishpitei Ribis p. 342; Ateres Shlomo 1:65 of Rav Shlomo Garelitz; Chut Shani Ribis 16:115 of Rav Garelitz; Vayaan David 2:219; Rav Z.N. Goldberg in Techumin 12:295 leaves this issue in doubt]

[6] Poskim in Sdei Chemed Lamed 141:24; Or Li 31; Shoel Umeishiv Reviah 3:29; Karban Elitzur Avoda Zara p. 137; Poskim in Kaf Hachaim ibid; Poskim in Piskeiy Teshuvos 11:19 footnote 132; See Sichos Kodesh 5710 p. 73-4;

[7] The reason: As a Rabbinically valid form of acquisition is valid likewise for biblical matters. Furthermore, the agreement for one to pay on credit is itself considered like payment from a Biblical perspective. Furthermore, the Chatzer is Koneh. [Sdei Chemed ibid]

[8] Sdei Chemed ibid; Rebbe ibid in Toas Menachem

[9] Morah Baetzba Kuntrus Kaf Achas 3; Shaareiy Teshuvah 658:5; M”B 658:10; Alef Lamateh 658:28; Kaf Hachaim ibid; Piskeiy Teshuvos ibid

[10] Sefer Haminhagim p. 139 [English]; Custom of Rebbe Rayatz, stated by Rebbe in Toras Menachem 5710 p. 85,  that he would pay only sometime after Sukkos, after receiving after receiving a detailed protocol of the Esrogim [There the Rebbe writes that in the first year of his arrival to America he requested from the Rebbe Rayatz to pay Rabbi Yisrael Jacobson before Sukkos for the Esrogim that he brought, in order so it will be completely his. The Rebbe however did not wish to pay right away. The Rebbe Concludes there “It is understood on its own who’s wishes were followed.” In the Sefer Yimei Melech page 163 it relates that the Rebbe Rayatz replied to the Rebbe by asking in a rhetorical fashion whether Rabbi Yisrael needs money for the holiday and from this the Rebbe understood that the Rebbe Rayatz was particular not to pay prior to the holiday.] See Otzer Minhagei Chabad 14:2

[11] Otzer Minhagei Chabad 14:3 “However, the Rebbe himself was particular to ask those who brought him the Daled Minim for their price and he would pay them right away”; Testimony of Rav Levi Bistritzky who personally brought the Rebbe the Lulavim and Hadassim;

Other records: Some claim that the Rebbe would pay for his Daled Minim only after the holiday of Sukkos. [Maaseh Melech Leket Chodesh Tishreiy 16 in name of Rav Leibal Groner “He would not pay for the Daled Minim until some weeks after Tishrei with exception to those Esrogim that he received from individual people in which case he would pay immediately.”]

[12] For example, see here where the Rebbe paid with cash: https://613tube.com/watch/?v=qX2XAV-rnJk

[13] See M”B 11:27 that if the seller is “Ayel Venafik Azuzei” then it is stolen; Choshen Mishpat 190:10-16; Aruch Hashulchan 11:22; Piskeiy Teshuvos 11:18