Employer paid excess salary for 11 months can they recover the amount

My employer miscalculated my salary and paid excess salary for 11 months. The salary slips match the amount paid, even the tax slip and income tax is paid on the higher salary. Now after an year, they say that excess amount was paid. No written communication from the company, forcing me to sign a declaration that I will get a certain amount deducted monthly to repay the excess amount.I wish to ask: When salary slips are matching the amount paid, what is the basis of recovery? How could 11 months salary slips go wrong. Why there is no written communication? Can they bind me from quitting?