Drain cherries, reserving liquid. Place cherries in a heatproof bowl. Bring 2/3 cup reserved cherry liquid and sugar to a boil in a small saucepan, swirling until sugar is dissolved. Continue cooking until reduced to about 1/4 cup, about 5 minutes. Stir in liqueur, then pour mixture over cherries. Let cool completely before refrigerating, covered, at least 1 day and up to 3 days.