Seven Days' Leave (1942) on Netflix

Soldier Johnny Grey is engaged to marry singer Mapy Cortes, but his plans go awry when he learns that he is the heir to $100,000 from his great-grandfather -- a bequest that comes with a catch: before claiming the money, Johnny must marry a descendant of his great-grandfather's Civil War enemy, General Havelock-Allen. Not wanting to disrupt his planned marriage to Mapy, Johnny must figure out how to concoct a temporary marriage-of-convenience with the descendant -- who turns out to be the beautiful Terry Havelock-Allen.