All by Myself (1943)

Career woman Jean, almost a partner in Mark's advertising firm, has been falling in love with Mark who is unaware of it. But unknown to Jean, Mark has become engaged to singer Val. After Jean finds out, she tries to save face by saying that she is also engaged, and then uses a little social blackmail to get psychiatrist Bill Perry to pretend to be her fiancé for an evening out with Mark and Val. Of course it backfires, things get more and more tangled, people fall in and out of love, and finally it gets sorted out into two happily married couples.