Hello, Amiroo.
As JoxerTM has posted above you're not allowed to give your password or account to anyone else.
Now, on to the quest. For this quest you are required to have $200 or more on pocket( I did this quest with $2750 on hand.) Wait until 7pm Which is 19:00 Server Time to see the Server Time look on the bottom right of your screen when the west is opened. It's right under speak. If it is between 19:00 and I believe 23:50 you can do the quest make sure you do not have a weapon equipped. Check back with the Sheriff and see if a new quest popped up. If it has, you must beat the Thief without a weapon on. If you fail to beat him, try adding some Dueling Gear or adding points into dueling.
If you cannot get the quest to work for some reason, perhaps you finished it before and you are currently on the other part of it after getting 5 silvers you give them to the sheriff and you must wait until Wednesday Server Time in order finish the next part of the quest which requires you to beat the dealer from which you get the Ring.
To see completed quests go to the sheriff and look under completed quest and look for the following:
Kate's Wedding Band (The Thief)
Kate's Wedding Band (Silver Mining)
If those are completed, you're on the last part which requires you to wait until Wednesday Server Time.