What constitutes 'good' (khayr) in the context of Mukataba?

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The definition of 'good' (khayr) upon which the obligation or recommendation rests is interpreted diversely but centers on trustworthiness and capability. Ahmad defined it as truthfulness, righteousness, and the ability to fulfill the stipulated payment for freedom. Others offered similar interpretations: Ibn Abbas mentioned wealth (Ghina) and payment of the fee; Mujahid specified wealth and fulfilling the obligation; Al-Nukha'i stated truthfulness and fulfillment; and Al-Shafi'i cited strength in earning capacity and trustworthiness.