The Demon’s Mistress by Jo Beverley

Lord Vandeimen returns home from Waterloo to ruined estates, his family all dead. His attempts to recreate something of his heritage leave him deep in debt, and he is ready to commit suicide when a woman bursts into his room with an extraordinary proposal.

Mrs. Maria Celestin, widow of a wealthy foreign merchant, will pay him a small fortune to pretend to be her betrothed husband for six weeks. Van cannot refuse, but he is wary of this outrageous good fortune. He is also intent on getting into the widow’s bed. He doesn’t expect to be dragged back into life, and into love, or into a battle to win the woman who bought him.