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?
I definitely agree that this is often more powerful than a lab. However, it doesn't make any sense to buy Coppers to fuel SoM... the turn that you play this, if you actually do give a Copper, then you haven't just played a lab... you've played a cantrip with extra cycling. Your handsize will NOT increase. So it's only advantageous if you're using it to thin out your deck, or to hurt the other player. Buying Coppers just to allow you to play SoM more means that playing SoM doesn't improve your deck OR your turn.
*Edit* RTT actually makes a good point, and similar to what I was saying... this is more like Spice Merchant than Lab.