Is the debtor entitled to withdraw from the sale of an apartment that was made under duress, and to pay the agreed-upon compensation, given that the value of the apartment far exceeds the debt, and that completing the sale would prevent him from paying other debts?

What you mentioned about taking money from people to invest for them, with a fixed annual return and a guarantee of their capital, is forbidden. This is because the Mudarabah contract is vitiated by guaranteeing the capital and a known profit. Mudarabah is a partnership in which the capital provider participates with their money, and you with your effort. The agreement must be on a common share of the profit, and the loss is borne by the capital provider. You do not guarantee the capital unless there is transgression or negligence on your part. If the Mudarabah contract is vitiated, all the profit belongs to the capital providers, and the worker is entitled to a fair wage for their work. As for the sale of the apartment and then retracting from it, the nature of the contract is not clear. Therefore, the details of the transaction must be clarified precisely to scholars.