off-plan

Al Salaam Alikum,I was wondering if someone can explain to me why or why not it its permissible to trade in off_plan properties.Here in Dubai many people buy properties off plan with a minimum down payment at what is called prelaunch or launch deals. i.e at the point where the developer declares their intention to build a development. To entice the buyers to purchase a property he then announces a prelaunch or launch deal where the buyer pays a small percentage of the property at a set price.  The new buyer enters an agreement to buy the property on a payment plan where the full price of the property is paid over the course of the construction and the date of the payments is set either by time period ( 10% every three months for example) or by milestones ( X% when the building reaches Y number of floors) and the remaining balance is paid upon delivery. The property is then considered delivered and received once all the terms of the contracts have been fulfilled.To my limited knowledge I can't see anything wrong in the above but the issue where people are raising eye brows is when people enter into these kind of agreements with developers and sell these agreements on to the next buyer.  So lets say there is an apartment advertised at prelaunch deal for $100,000  with a 10% down payment.  I put in the 10% down payment and I sign the contract to pay the rest during the construction and development of the building over the next 2 years.   But before the construction has even started the property value has changed ( according to new market prices) so I put up the property for sale although I have neither fin ished the payment nor has the building construction began.Legally speaking there is nothing stopping me from this practice. But asking around I get a variety of opinions on the permissibility of this practice.Please advice and Jazakum Allah Khyran.