Hi Flavio. I have a decent collection of Ottoman coins from the time of Orhan (abt. 1325 AD) till the last Sultan Muhammad VI (1923 AD), altogether some 600 – 700 different pieces. So I feel free to identify your coin. First of all, I like to state that the coin is a very good help for dating your dagger, as it is obvious that the assembly date can not be earlier than the date of the coin. (nevertheless, some parts like the blade might be much older). Your coin is indeed from the Ottoman Empire, it is a 5/10 Quirsh minted in Misr ( Egypt), so minded not far from Sudan. The side with the Tughra of Sultan Abdul Hamid II. is visible, below there is the Arabic number 21 and the word Sene, which means year. So this coin was minted in the 21st year of the reign of this Sultan. He reigned from 1293 AH till 1327 AH, which is from 1876 AD till 1909 AD. So the year of minting was 1896 AD (you have to add 20, not 21, as that would count the first year twice).
On the backside, you will find a large Arabic 5, date of accession 1293 and the name of the mint Misr. I only have to disappoint you about the material, it is not Silver, but Copper-Nickel, the silver coins from this Sultan have much different ornaments.
A beautiful dagger it is indeed, congratulations!
