A valid sale contract becomes binding once it is concluded
1-2-2010 | IslamWeb
Question:
My partner and I opened a store 5 years ago. The store was not profitable. the store was put for sale 2 sepeate times. No one made any offers. I gave my partner a choice to buy the store. He refused and I bought my partner's share in a store in Febuary 2009. I told my partner that I will be closing the store in May 2009 once the lease expires. And transfer all the customers to my other location 2 miles down the same street. He agreed and I paid him in full. On april 1st 2009 cigarettes went up in price by 30% and the voulume went up and the store became profitable. The lease expired in May 2009 and I continued to sell the product. 2 months later I renewed the lease for 5 more years. In October 2009 my partner said that I have to pay him for what the store is realy now worth. He says that he only sold because the store was going to be closed otherwise he would have waited till the lease expired. I say that he already sold and got paid in full. My intention was to close but things changed. I decided to take a new risk after the fact and signed a new lease. I also believe that the value of the store at time of the sale was worth it for him and he agreed to it. What do you think. He is my freind but I don't want to buy the store twice.
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
It appears from the question that the sale was concluded fulfilling its conditions and pillars, so your friend has no right to ask you for the additional payment that you mentioned in the question and he has no right to withdraw from this sale contract because the sale is binding on both parties of the contract once it is concluded and it cannot be invalidated except with the agreement of both parties.
If you are pleased to rescind the contract, then this is Iqaalah (recession of contract with the free consent of both parties), and this is something desirable. The Prophet said: "If anyone rescinds a sale with a Muslim, Allaah will forgive his slip [on the Day of Resurrection].” [Abu Daawood, Ibn Maajah, Ahmad and Al-Albaani]
It should be noted that selling cigarettes is forbidden and its profit is ill-gotten money; for more benefit, please refer to Fataawa 82827 and 84634.
Allaah Knows best.