What is the ruling on contributing a sum of money with a group to a contractor, on the condition that the contractor invests the money to return, after 5 years, a residential apartment whose value is three times the amount paid?

There is no objection to acquiring the apartment in this manner, as this falls under the contract of Istisna' (manufacturing/construction). The conditions for this contract are that the apartment's description, location, area, and form must be specified in a way that eliminates any ambiguity, along with determining the delivery date. It does not matter if its current price is higher than its price in the Istisna' contract.