A quarter-century ago, a young banker named Andrea Orcel led a team at Merrill Lynch that cobbled together more than a half-dozen midsize banks to create Italy’s No. 3 financial firm. Over the following decade, the company now known as UniCredit SpA spent $65 billion buying lenders across central and eastern Europe to become a regional colossus—and Italy’s biggest bank.

Orcel went on to take the top job at UBS Group AG’s investment bank. Then came a courtship with Banco Santander SA, which ended in a court ruling that could net Orcel as much as €43 million ($47 million) after the Spanish lender withdrew an offer for him to become chief executive officer.