I found a $20 bill on the floor of the beis medrash.

Q: What am I obligated to do with the money?

A: The halachah is yei’ush shelo midaas, lo havei yei’ush. This principle addresses lost objects that the owner does not realize are lost, but undoubtedly once he realizes they are lost, he will despair of recovering them, e.g., an object lacking a siman (identifying characteristic). Even later after the owner realizes the object was lost and despairs, the finder remains obligated to return it.

The Gemara (B.M. 21b) challenges this principle based on the Mishnah’s ruling that one who finds lost money may keep it, seemingly without concern that the owner is unaware that the money was lost. The Gemara answers: Since people regularly check the money in their pockets it is assumed that the owner immediately realizes when he loses money (C.M. 262: 3, 6).

Chazon Ish is cited as stating that the halachah is an enactment of Chazal. Consequently, although nowadays people do not check the money in their pockets as frequently, and often do not know how much money is in their pocket, nevertheless the enactment remains in force and the finder may keep the lost money (Pe’er Hador, vol. III:98; Maaseh Ish, vol. III:29, 87 and vol. VII:163. See also Mishpat Haaveidah, p. 93).

Some authorities question this assertion since the implication of the Gemara and Poskim is that the halachah is based on the presumption that the owner realizes the money is lost and despairs of recovering it, and it is not an enactment of Chazal that applies automatically.

Other authorities write that nowadays, since people do not realize when they lose money, the principle of yei’ush shelo midaas applies (Hashavas Aveidah K’halachah, p. 163 quoting Harav Elyashiv, zt”l; see also Taz, C.M. 262:6 and Pischei Choshen, Aveidah 2:[32]).

Others contend that although yei’ush shelo midaas does not allow the finder to keep the lost item for himself, he is permitted to use it until he finds the owner and returns it (Shach, C.M. 260:26 — according to Rambam; see also Sema 260:42 and Rav Akiva Eiger for an alternate approach). Others reject this approach since the matter involves a Biblical prohibition and rule that one may not use an item found before the owner despaired of recovering it (Shulchan Aruch HaRav, Metziah 2).

An approach that satisfies all opinions is to record all the details related to the lost money (e.g., the amount, where it was found, the date it was found and anything else that would allow the owner to prove that it is his) and then the finder may use it, understanding that he will return it to the owner following witnesses’ testimony, or when Eliyahu Hanavi arrives (Mishpetei HaTorah, B.M., Hashavas Aveidah 2:13). Obviously, this is limited to amounts the finder would have available to give to the owner, but if the finder would not be able to produce that amount when requested, he may not employ this approach (see C.M. 267:25; Hashavas Aveidah K’halachah 6:3).