You Belong to Me by Johanna Lindsey

In all the world, no man exists who can tame Alexandra Rubilov...

A fiery and beautiful free-thinker, Alex's steadfast refusal to marry has frustrated her hapless father. And so he creates a "long-forgotten" agreement and sends his rebellious daughter away, ly maintaining that Alexandra has been promised since childhood to the handsome, insufferable libertine whom she must now accompany to his homeland to wed.

Dismayed to find himself suddenly engaged, Count Vasili Petroff plans to repulse his unwanted fiancee by acting the perfect cad, unaware that wily Alexandra plans to follow a similar path. But the road to deception is a rocky one and its many unexpected turns can lead two reluctant companions to a most unanticipated destination: that place called passionate love.