One makes an exact carbon-copy (perfect copy, in such a way that one will never be able to tell the difference between an original and a copy. For arguments sake, lets say through witchcraft) of (say) a work of art and destroys the original.
He then proceeds to sell this copy for the cost of an original work of art (say $1,000,000).
If he would have informed the buyer that this was just a copy, he would have only been able to make (for example) $5,000.
Does he have to (between him and Hashem, as nobody can prove it to be a forgery) pay back the difference? (He probably isn't allowed to misrepresent his work in the first place, but ex-post-facto is he allowed to keep the money)?
On one hand, inherently the painting isn't worth nearly as much as the selling-price, yet on the other hand it is worth that amount on the open market.
Based on comments from Is there any issue except for Dina Dmalchusa in counterfeiting money?