Is the sum of 20 Bahraini Dinars (approximately 15 Kuwaiti Dinars) found in the currency exchange market considered of a value that necessitates holding it for a year, or is it permissible to benefit from it until its owner appears after reporting it?

It appears that this lost item is something its owner would pursue and has value. Therefore, you must publicize it for a year. After that, it is permissible to use it, provided you guarantee it to its owner if he comes and describes it. This is because the most authoritative view among scholars is that there is no specific legal estimation for a lost item that becomes owned merely by taking it without publicizing it. The Hadith of Zayd ibn Khalid is general for all lost items, except what is excluded by evidence. Specification and estimation are not known by analogy, but rather by text or consensus. However, it is permissible to take what the Prophet, peace and blessings be upon him, permitted, such as a staff, a whip, a rope, and items of similar value. What you did, searching for its owner and informing those you know, does not suffice for the obligatory publicization.