It is narrated from Imam Amir Sha'bi, may Allah have mercy on him: A slave died and his free father and his sons (i.e., the deceased's sons) from a free woman are present. So, for whom will the wala' (right of inheritance/freedom) be? He said: In such a case, the wala' will belong to those who own the grandfather.
Hadith Referenceسنن دارمي / من كتاب الفرائض / 3200
Hadith Gradingتحقیق (حسین سلیم أسد الدارانی):إسناده صحيح إلى الشعبي، [مكتبه الشامله نمبر: 3211]
Hadith Takhrijاس اثر کی سند شعبی رحمہ اللہ تک صحیح ہے، اور ابونعیم کا نام فضل بن دکین ہے، اور زکریا: ابن ابی زائدہ ہیں۔ دیکھئے: [البيهقي 307/10]