Substitute bride

From the back cover of the 1983 Harlequin edition:

"I'll marry you instead," he told her Rick Conway would go to any lengths to get his revenge on Emma's beautiful but faithless cousin. And though Emma was thunderstruck and offended at his cold proposal, she soon overcame her scruples. It would be a temporary arrangement, Rick claimed, and it would be a way of escaping her life of impoverished drudgery. All Emma had to do was sustain her dislike of him. And that was when her problems began!