What is the minimum amount of money that necessitates purification (zakat) for a year, and does that include 50 Riyals?

A small lost item (luqatah yasirah) may be utilized by its finder, and there is no obligation to announce it. There is no disagreement among scholars regarding the permissibility of taking a small lost item and benefiting from it. The distinction between a significant item (jaleel) and an insignificant, small one (tafih yasir) is determined by its value according to custom and the common practice of people in the locality. It is not possible to definitively state whether an amount of 50 riyals is considered a small lost item that may be utilized without announcement, or if it is an item that requires announcement. The prevailing custom of the place where the money was found should be referred to, and the local scholars there can be consulted.