Q. From the Shulchan Aruch Siman 266 Seif 13, I have the following question:
If the event that one finds a lost object that is not muktzah in an area surrounded by an
eruv where it is clear that it is permitted for the finder to take possession of the lost object for himself, would it be permitted to pick it up on Shabbat or Yom Tov to take it home? (for example, I found unmarked sunglasses in the middle of a field where most residents nearby are not Jewish).
If not, would it help if I have in mind not to truly take possession of it until after Shabbat?
If not, can I kick it to a safe & hidden location in order to retrieve it after Shabbat?
Any difference for Sephardim?

A. Rashi (Beitza 37a) quotes two reasons why no buying or selling is permitted on Shabbat. It is due to the Rabbinic prohibition based on the posuk (Yeshayahu 58: 13) “Memtzo Heftzecha,” meaning refrain from engaging or speaking about weekday activities on Shabbat. Another reason is lest one will come to write down the details of the transaction. Ramban (Vayikra 23:24) adds that this rabbinic prohibition is founded on the biblical mitzva of “Shabaton” which refers to resting on Shabbat.
Based on the above Mishna Berura (306: 33) prohibits giving presents on Shabbat unless the gifts are needed then. (See also Shemiras Shabbos Kehilchoso (29: 31.)
Horav Shlomo Miller Shlit’a opinion is that indeed it is better to have in mind not to acquire the found object on Shabbat. Same ruling for Sephardim or Ashkenazim.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a