The Philadelphian: by Richard Powell.

This book is about a young lawyer whose boss's daughter falls in love with him, and the young one is persuaded by his boss to postpone the marriage a few years, thus winning - the daughter wanted to marry now! Sure enough she marries someone a couple of years later, and the young man has learned a lesson about how the old one played him.

He gets his own back, incidentally and inadvertently, when he informs an old woman and her granddaughter that they could save money in taxes if the firm informed them of one particular point, and the old lawyer's firm is up in arms against him accusing him of stealing a client and challenging him to show another point or lose his license - and he wins that one, after a couple of nights or so of poring over the accounts. The old girlfriend, now a mother of two, sends him her congratulations.

The young lawyer goes on to marry the young heiress, with the blessings of her grandmother who with her estate is now his client, and goes on to achieve more success in life and career, trumping the old fox who had prevented him marrying his daughter, trumping him more than once.