There are no opinions involved when it comes to logic.
the customer walks in
you give him the ring, you lose $70
you go to the baker, get $100
give the customer $30, you lose $100 total
you have $70 cash left from the $100 the baker gave you
you give the $70 + an extra $30 back to the baker
you lose $130
There is no such thing as a potential cost, just like there is no such thing as a paper loss. A loss is either realized (cash) or unrealized (security or item exchangeable for cash) but both forms of loss are "real". Just because someone hasn't exchanged the item for cash doesn't change its value.
But what irritates me here is that we're assuming that the jeweler would have otherwise sold the ring. He at least lost $30 + value of the ring - we don't know anything else.
$30 plus what he could have sold the ring for, yes. I don't think we're saying different things, I just think it's reasonable to assume the words "$70 ring" mean $70 is what he could have sold it for
30 bucks, and the ring.