ANSWER:
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 1. Tell the vassals to line up with their sacks.

 2. Number the sacks sequentially from 1 to 10.

 3. Take out a number of coins equal to the sack number. That is, take 3 coins from
    sack #3, 7 coins from sack #7, etc. You should have 55 coins in your hands.

 4. Place the coins on the scale.

 5. If all the coins were normal, the scale should read exactly 55 ounces. However,
    because there is at least one shaved coin in the pile, the weight will be
    slightly less.

 6. The number of the sack containing the shaved coins is equal to by how many
    tenths of an ounce the weight of the coins is lighter than 55 ounces. So, if the
    scale read 54.7 oz, the vassal who brought sack #3 is the dishonest one.


 P.S. The actual number of coins in each sack was irrelevant, as long as there were
 at least 9 coins in each sack.