For banks, lucrative transactions often pay so well because they carry high levels of risk — meaning, they may end up not being lucrative at all. Some of the world’s biggest banks are living that reality after having to sell, at discounts as high as 35%, much of the roughly $80 billion in “hung debt” they raised to facilitate mergers and leveraged buyouts before deal activity slowed to a crawl. The only thing worse than selling debt at a discount is not being able to sell it except at a level co