Without possessive yāʾ: غُلَامٌ، غُلَامًا، غُلَامٍ — the case endings are visible.
With possessive yāʾ: غُلَامِي — the surface form stays the same in all three cases.
غُلَامِي is still inflected, not indeclinable; its case is estimated.
Identify its case from the sentence, not the visible kasra alone.