It was an offer that has rarely been made by any banker caught up in a scandal so early on: to give back all the money he made without having a plea deal in the bag. Last Tuesday, Co Down-born investment banker Martin Shields offered to repay almost €15 million he made from so-called cum-ex transactions when working with some of the world’s biggest banks and wealthiest investors.   Shields is the first banker in the world to offer to return the money made from cum-ex, and he may not be the last in an evolving financial scandal.  Hundreds of other…