According to al-Shaafi'i, Abu Thawr, Ibn al-Mundhir, and Ibn Qudaamah an oath from a kaafir is valid. He is obliged to the kaffaarah if he breaks is. This applies whether he broke the oath while he was a kaafir or after becoming a Muslim. The evidence for this is the following:
Narrated Ibn `Umar:
that `Umar had vowed in the Pre-Islamic period to perform I`tikaf in Al-Masjid-al-Haram. (A subnarrator thinks that `Umar vowed to perform I`tikaf for one night.) Allah's Messenger (ﷺ) said to `Umar, "Fulfill your vow."