What is the ruling on preemption if the portion was purchased for a debt price (in the seller's liability) which later turned out to be invalid?

Primary text

If the portion was purchased for a price due against the buyer (in the 'dhimmah), and then the price becomes due but the portion is found to belong to another, preemption is obligatory because the sale itself is valid. If the seller cannot collect the price from the buyer due to insolvency or other reasons, the seller has the right to rescind the sale, prioritizing the preempter's right. This is because the preemption secures for the buyer what he can use to pay the price, thus alleviating his hardship, reconciling both rights, and is therefore preferable.