Musannaf Ibn Abi Shayba 36:55

Sufyan ibn 'Uyaynah narrated to us, from Isma'il, from Qays, who said: Abu Bakr purchased Bilal—meaning [from his masters]—for five awqiyah [of gold/silver] while he was being buried under stones. They said: "If you had refused [to pay] except for one awqiyah, we would have sold him to you [for that]." He said: "If you had refused [to sell] except for one hundred awqiyah, I would have taken him."