Must one pay for an esrog or arovot before succot. Most likely every retailer will tell you if you dont pay you are not yotsai. The question really is has he paid his wholesaler and has the wholesaler paid the farmer or grower something very unlikely before yom tov and before he has sold them. So is one yotsai.
Sefer Haminhagim (Chabad) writes that "One does not necessarily have to pay for the Four Species before Sukkos".
"See the Acharonim on Shulchan Aruch, Orach Chayim 658; Sdei Chemed, Aseifas Dinim: Daled Minim 3:2; ibid., Klalim, Lamed 141:24."
Per this: When the Lubavitcher Rebbe came to the previous Lubavitcher Rebbe and suggested he pay the Arba Minim seller before Sukkos, the previous Lubavitcher Rebbe asked "Is he in need of money for Sukkos?" The Rebbe understood that the previous Lubavitcher Rebbe would specifically not pay before Yom Tov; however, he himself would always pay right away for his Arba Minim.
See Sefer Nitei Gavriel perek 46:1,20 which discusses this with many sources.In short there might be a problem of a kinyan min haTorah which is absent without payment.There is a Rabbinic kinyan of mishicha,but we want lachem for the mitzvah. It is best to be fully paid before Yom tov.
A check is like money ,but a post dated check is not so clear,some retailers feel happy with it while other not,every situation is different,if he knows you or not,see Shevet Halevi 7:83.