What is the ruling when one partner in jointly owned wealth sells his share to his co-owner midway through the Hawl?

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This scenario is the reverse of the first issue: where there was self-mixing, it became mixing with a stranger, and here, where there was mixing with a stranger, it becomes self-mixing. Following Abu Bakr's opinion, the owner owes nothing until the Hawl is completed on the combined wealth from the time he gained ownership of both parts, unless one part individually reaches the Nisab.