In what ways was the Buddhist theory of a social contract different from the Brahmanical view of society derived from the Purusha sukta?

The Brahamans often cited a verse from the Purusha Sukta in which they define the sacrifice of the Purusha, the primeval man. They said that the four social categories have derived from the body of the Purusha. The Brahamans are obtained from the mouth of the Purusha, the arms made the Kshatriyas and thighs made the Vaishyas. From his feet there was origin of Shudras.