وَإِذَا حَصَلَ الْإِيجَابُ وَالْقَبُولُ لَزِمَ الْبَيْعُ.
Translation: When offer and acceptance are completed, the sale becomes binding.
“The sale becomes binding” means the completed valid agreement cannot be cancelled unilaterally simply because one party changes their mind.
A specific book sold for 500 rupees with valid acceptance becomes binding. Consensual cancellation (iqalah), or a recognized legal option, is a separate matter.