Detailed Explanation of statement of Abu Yaala ibn al-Faraa' Al Hanbali in al-Ahkam As Sultaniyyah

ويجوز شراء أولاد الْحَرْبِ مِنْهُمْ، كَمَا يَجُوزُ سَبْيُهُمْ. وَيَجُوزُ شِرَاءُ أولاد العهد منهم، ولا يجوز سبيهم. كتاب الأحكام السلطانية لأبي يعلى الفراء Page 145 Sorry for asking again same question but couldn't get proper clarity regarding his statement. What does Imam Abu Ya'ala ibn al-Faraa' al-Hanbali mean by saying it is permissible to buy from Ahlul Ahad but not permissible to capture a proper detailed explanation in a simplified manner? Regarding Ahlul harb it is clear it is permissible to buy and capture them but this Ahlul 'aahad creates confusion due to the word "ولا يجوز سبيهم". Does Imam mean that it is permissible to buy and set free or he mean if Ahlul 'aahad sell them as slaves it is permissible to buy and keep them as slave but capturing and enslavement are prohibited?