Is participation in a water desalination plant project, where part of it is financed with the financiers receiving double what they paid over three years in monthly installments, considered usurious, especially since they demand their principal back after recovering nearly their initial capital?

If the payment was in the form of a loan, then stipulating an increase is usury (riba), and they must repent, and they are only entitled to their principal amounts. However, if it was under a Mudarabah (definition) (profit-sharing) and partnership contract, then the contract is invalid, because it is not permissible to set the partner's share as a specific number of dirhams. The contract must be annulled, and the profit belongs to the owner of the capital, while the one who worked in it is entitled to a fair wage (ajrat al-mithl).