Notice period prior to resignation doesn't alter the right of an employee to get his salary for the services he rendered to the employer or the employer's liability to pay at the end of the wage period.
When the employee is present for work for all the days of work during the notice period, any shortfall noticed by the functional head which can lead to a dispute would be a question of fact depending on the nature of the work particularly when its accomplishment being a matter of joint efforts of many people or involving cyclical phases. Therefore, certainly, it cannot be a valid reason for extension of the predetermined notice period of an employee whose resignation was accepted and the date of actual exit already agreed by the employer.
Apart from being a breach of the contract of employment, holding the salary for the notice period consecutively is neither a good HR practice nor an act of legality which could be supported by any plausible explanation by the employer.
Similarly, it is not a good administrative practice on the part of a prospective employer to shorten the time of joining of any candidate already employed in another organization.
You should note the point that paid employment career is a continuous process facilitated by cordial inter personal relationship with your colleagues particularly with your immediate superiors for they are the best judges of your performance. So, please don't indulge in wild imaginations which can make you more easily emotional rather than rational to handle a difficult situation adeptly. Still there is time to patch up things.
Therefore, whatever the way you desire to make the separation from the present organization, it is better in the interest of your career that you should, by all means, ensure that it does not create a conflict situation which can adversely affect your career at the end.
From India, Salem