Thank you for the reply , i have already found a solution and fixed this , I'll post it here for people who are also facing similar problem.

account_cancel module doesn't need to be installed on Odoo V10 or above , so all we gotta do , is head over to 

Accounting -> Configuration - > Accounting - > Journal: here click the journal which you want to cancel out , for customer invoices it's usually Bank accounts , on the bank or any similar journal page , click on Advanced Settings and check the tick box for " allow entries to cancel each other " 

now head over to Specific customer whom's balance you are sure to be overstated , open the specific invoice , down below the registered payment : - > click on  ' ? ' on the left side of the wrong payment and click on open and there you will be able to cancel that entry but still in order to delete it , you'll need to un-reconcile that payment and then head over to that entry gain and delete it. 

Done , That customer statement should be fixed and wrong payment should be removed by now.