Who gets the money?

[RickJ]

Apparently  I don't see what you see Mills.   Both people know the money is not theirs and that it's very likely that the owner will turn up sometime to collect what is rightfully theirs.  It would be foolish to believe otherwise and stupid to act on such a belief  such as by spending the money.   

Mr B's credit card account is no business of Mr A. regardless of the nature of their relationship.  It's the same question as asking "Mr B lends Mr A $100 dollars to buy shoes.  Mr. A repays the loan and Mr A uses the money to place a bet on "MyBigHorsey".   MyBigHorsey wins  and pays a 100:1.  How much should A share with B???".  It makes no sense to even think about it.     

[SSH]

Having had accidental credits to my card and then had them "corrected" later, I think its very likely that SC will try and get the money back from B, so B has a big risk in giving A anything.

[MillsJROSS]

Rick J, I agree with you, which is why I said I would have tried to return the money to the Shoe Company. However, the premise of the question was that option 4 wasn't really an option and the mistake was never going to be caught, so I assumed our two characters were morally ambiguous in there decision making.

I don't, however, agree that Mr. B's credit card isn't any business of Mr. A's when we're talking about Mr. A's transaction. What if the opposite happened and the shoe company charge more than the $100? Any discrepancy with the bill should get the attention of Mr. A.

The split is done because Mr. B revealed the mistake to Mr. A. Since they both were responsible for the situation occurring and desire to continue a friendship, they split it down the middle. If the Shoe Company comes calling later both parties pay. We know Mr. A is responsible enough to pay back previously, so we can assume he won't run out on the bill next time.

-MillsJROSS