He didn't lose money, he sold something at a smaller profit (we assume he's intelligent enough that he didn't cut his margin down beyond cost). The reason it's a win for him is that it's a sale he wouldn't otherwise have made and therefore a profit he wouldn't otherwise have had.

There are potential downsides (if it was his last one he might have missed the chance to sell it for full price, it might be seen as setting a precedent) but generally salesmen understand these and figure them into their reasoning.