All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Allah, The Almighty, has legislated sale and laid down certain rules and conditions for it in order to achieve stability in various transactions and prevent dispute amongst people. One of these conditions is that offer and acceptance should be done immediately without laying down conditions or deadlines.
In conclusion, the sale mentioned in the above question is invalid because it violates the conditions of Sharia. It is almost like the prohibited sale on credit which is employed to obtain usury, because five thousands are lent in return for seven. And Allah knows best.