Unethical Landlords

[gazumped]

If a landlord quotes you a price for a lease extention, and makes the price valid for 6 months, is he at liberty after only 4 months has elapsed - and whilst the purchase is proceeding - to cancel his original asking (and agreed) price, and demand an additional 40% ? If 'yes', how can anyone be sure at the beginning of a purchase what the eventual cost of the extention will be ? If 'no', what is my remedy ?

[almcd007]

This question will be better answered if posted in the Law section as this is the adverts section.