Hasan al-Basri (may Allah have mercy on him) said: "Whoever does not find a slave for the expiation of zihar and fasts, then becomes wealthy, he should leave the fast and free a slave." Later he said: "He should complete the fast and not free a slave."
Hadith Referenceسنن سعید بن منصور / كتاب الطلاق / 3288