I Will Repay (Paperback) (ISBN-13: 9798897186860)

I Will Repay (1906) by Baroness Orczy:

Set during the French Revolution, the novel follows Juliette Marny, who swears vengeance against Paul Déroulède after her brother is killed in a duel with him. To carry out her oath, Juliette enters Déroulède's household, intending to betray him to the revolutionary authorities. However, she discovers that he is a noble, selfless man devoted to justice and to protecting the innocent. As Juliette falls in love with him, her loyalty is torn between her vow of revenge and her newfound feelings. When Déroulède is arrested and faces the guillotine, Juliette must choose between her oath and her heart.

It's one of the Scarlet Pimpernel series stories, blending romance, intrigue, and themes of honor, love, and sacrifice against the backdrop of revolutionary France.