Giving charity and spending money on Motzei Shabbos:
It is our custom to avoid giving money or buy items on Motzei Shabbos, as doing so gives nurture to the Kelipas. It is however permitted to distribute charity on Motzei Shabbos, as by doing so there is no better way to drive away the Kelipas.
 Shaareiy Halacha Uminhag 1/160; 5/35
 This was the custom of Rebbe Rashab, as revealed by Rebbe Rayatz. See Sichas Motzei Shabbos Vayeilech 5739
Prohibition of Lo Sinacheish: In Shulchan Aruch Yoreh Deah 179/3 the Michaber rules that one who states he will not give money to a collector because it is Motzei Shabbos transgresses “Lo Sinacheish”. This raises a question regarding our custom to avoid giving money on Motzei Shabbos. The Rebbe mentions this in Shaareiy Halacha Uminhag 1/160 and concludes that nevertheless this was the custom. To note however from Taz 179/2 and Pischeiy Teshuvah 279/3 that if one does not tell the person why he is not giving the money and simply thinks in his head that it is because of Motzei Shabbos, then no transgression has been done.
May one pay somebody to whom he owes $, such as to pay the baal korei for leining on Shabbos? Does it make a difference if one pays cash or writes a check? May one buy items for a mitzvah, such as to purchase arba minim for Sukkos or food for melava malka (such as those who buy pizza on motzoi Shabos)?
When motzoi Acharon shel Pesach is motzoi Shabbos, may one buy pizza, as many are accustomed?