The Golden Collar by Elizabeth Cadell

Henry Eliot isn’t sure exactly how he became engaged to the beautiful and autocratic Marly Stonor, daughter of Sir Bertram Stonor, the CEO of the firm where he is an architect. However, his status as future son-in-law doesn’t spare him from being summoned like a schoolboy when Sir Bertram has an assignment. Sir Bertram has been thwarted in his attempts to purchase some coastal property in the Algarve region of Portugal for his daughter (and, eventually, Henry) and he decides to send Henry to represent his interests because Henry speaks Portuguese, having grown up in Brazil.