Judge Ahmad bin Muhammad al-Birti says: I asked Yahya bin Aktham about this hadith, and he said: This was specific to the Prophet (peace and blessings be upon him). The Shaykh (may Allah have mercy on him) said: This is also mentioned from al-Muzani (may Allah have mercy on him), that he mentioned this hadith to al-Shafi'i (may Allah have mercy on him), who took it as a special case. The point of the special case is that he freed her unconditionally and then married her without a dower, whereas the marriage of anyone else is never without a dower. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13743