I think this card is too powerful. It is actually stronger than Laboratory but it only costs $4.
How is it stronger than Lab? Well, it allows you to remove Copper from your deck AND junk up the player on your left. Sometimes this will be a disadvantage (possibly after all the Copper is gone), but usually it will be good for you. By the time it becomes a drawback, you should be able to work around it easily -- have a treasureless deck, or buy/gain extra Copper to fuel SoM.
Hm, actually, I can see how the drawback could actually hurt. If a treasureless deck isn't possible, it would usually be pretty difficult to keep getting enough Copper to fuel multiple SoM plays every turn. However, you could just buy a single SoM as a very powerful non-terminal deck-thinner, so it is still too strong.
Mechanically speaking, "give a treasure" is ambiguous. Does the player "gain" the treasure, or is it like Masquerade? Does the teasure go into their hand, or their discard, or somewhere else?