Mr. Rinyo Clacton's Offer by Russell Hoban

A heartbroken Londoner is approached by a suave, unsettling stranger who offers him a fortune now in exchange for the right to kill him later, a pact he rashly accepts. As he spends the money and tries to rebuild love and meaning under the shadow of a ticking deadline, he’s drawn into a seductive, menacing game that tests his will to live and the price of his soul, blurring lines between consent, fate, and the value of a life.