1) May one buy arba minim on credit (he will pay after Sukkos)?
2) Is it acceptable to pay for arba minim with a check? It could be that it will not be cashed until later and the four minim are not truly “לכם”… is this a concern?
It is not L’chatcila to buy the arba minim on credit to pay after yom tov. The reason for this is that since he didn’t pay for it yet he only acquired the arba minim m’drabonon, and even if he brings it into his house and is acquired that way, the M:B 658-10 says that it l’chatchila one should be careful to pay before Yom Tov. If a check was given, even if it was post dated, it is still alright. This is since the seller accepted it as his payment, it is considered as if it was paid for already and sold, and the money that is owed is like a loan. Therefore regarding the 4 minim he did acquire them.
Kovetz Halachos (R’ S. Kaminetsky shlit”a) Succos 18-13.