The coin has been "tampered" from its original state, like trying to polish patina off an old fiat coin. I'd only offer face value, but that's just me!
there are a good number of casascius coins that have more then their face value loaded. A good number of them have been subject to spam transactions from various people, often of .0001 BTC or less.
The casascius tenth generally will fetch a premium of 1.9 or so when graded MS67 and loaded with .1 BTC. I would think that the premium would decline as more funds are loaded onto it. The premium would probably be the same if there is one or two spam/dust transactions and would only decline slightly if a small amount of extra funds was loaded onto it.
Lesbian cow was obviously able to get the coin with a smaller premium then what they normally fetch. I am not sure what this coin is worth, probably not 1.75 BTC in my opinion, however it is certainly worth more then it's face value, especially considering that it is a) graded, and b) received a MS67 grade.
Unfortunately the physical coin market is illiquid so it is not always easy to sell coins quickly and if coins are to be sold quickly they will often sell at a discount to what they would normally otherwise sell for.
This is an interresting issue...
Should a Casascius 0.1 BTC contain only the original 0.1 BTC to be a 'pure' coin?
Because if so, if you have bad intentions, a coins can be easily devalued by putting more btc on the public address, which are all public.
IMO it does not matter how many bitcoins are on the coin, it is about the physical amount of coins that were made and the grading.
But still food for though...
Well the basis for my theory is that unfunded and redeemed coins tend to sell at significant discounts verses what coins that are funded at their face value. I don't think I have seen any sales of redeemed cas coins that received a high grade, however I would hypothesize that the premium for a redeemed coin would be less then an unredeemed coin that received the same grade.
I don't think your 'attack' would work very well, because I think the amount the premium would decline by would be less then the additional amount loaded onto the coin. I am just throwing numbers out there, however a coin that is loaded with an extra 1
BTC might see it's premium decline by .1
BTC, so the net price would be .9
BTC higher. The issue in this case however is that this appears to not have been an intentional 'attack' but was rather a mistake, and you are trying to make good on someone else's mistake.
I think my point is somewhat proven by the fact that you are essentially selling the coin for what is essentially the same price you paid, the day (or close to it) that you received the coin. I understand you to be a collector of cas silver coins, somewhat assume that you have a good amount of money, so if the coin really would maintain it's value when it is loaded with additional funds, then I would think that a collector would pay for the coin a second time if their payment was accidentally loaded onto the coin (if the coin would maintain the same premium then the net effect to your finances would be zero, and you would get to keep what is a very rare coin).
I know that if I were to be offered a premium of 1.25
BTC on a MS67 cas tenth, then there is a pretty good chance that I may not even respond to them as I would think they are not serious about buying.