Is the sacrifice (udhiyah) of an immature, non-pubescent boy, who bought a cow for sacrifice with his own money, valid according to the Shafi'i school of thought?

The Maliki school of thought holds that a child is not obligated to offer a sacrifice (udhiyah (definition)). It is permissible for the guardian to offer it on his behalf from the guardian's own money, not from the child's money. This is because the udhiyah is a voluntary act of charity, and the guardian is prohibited from making voluntary donations from the money of his ward. Furthermore, a child is not considered competent to make purchases.