Bound by Blackmail

In the Alcolar-ka ni Ai o series (アルコラール家に愛をシリーズ), also called "The Alcolar Family" series:

1. Tsumetai Hitomi no Yuuwakusha (冷たい瞳の誘惑者; Bound by Blackmail):
   He'll make her his mistress for revenge...
   Jake Taverner has never wanted a woman as he wants Mercedes Alcolar. So when she rejects him, Jake's hurt pride demands revenge!

...his mistress for a month!
Jake traps Mercedes into a fake engagement as he embarks on a skillful seduction. She'll share his bed for one passionate month, bound to him by blackmail....

2. Hiiro no Hanayome (緋色の花嫁; The Spaniard's Inconvenient Wife):
   The convenient solution is to marry her!

Ramon Dario desperately wants the Medrano Company. But the deal has an outrageous condition -- marriage to the notorious Estrella Medrano! Ramon won't be forced into anything, least of all a wedding with a scarlet woman!

But Estrella is not what he thought. Ramon can't get her gorgeous body out of his mind. He even starts to believe she may not deserve her reputation! A marriage of convenience -- and desire -- might be a good deal after all....

(info from fictiondb)