Wicked Earl Seeks Proper Heiress by Sara Bennett

Lady Averil Martindale, known as The Heiress, is searching for her lost sister. She is not thinking of marriage at all, but after seeing Rufus, the Earl of Southbrook, at the theatre, she informs the Husband Hunters Club that he is the man she is going to marry. With his dark, brooding looks and scarred face, Rufus is a social outcast. But when he rescues her from a tricky situation and then offers to help with the search for her sister, Averil finds herself enjoying his company more and more.
She might even be falling in love with him.
Rufus is desperately in need of an heiress. His castle is falling down and he is in dire financial straits. He knows what society thinks of him, and he has his secrets, but Averil might be just the wife he needs. And the more he gets to know Averil, the more he likes her, so the last thing he wants is for her to believe he is pursuing her for her fortune. When Averil visits his castle and learns the truth, their romance seems to have come to an abrupt end.
Or has it? Rufus decides to do everything in his power to save it.