1. As explained above, sec 80CCD(1B) is an additional benefit provided to people who like to make more investments in NPS.
2. It should be read in conjunction with sec 80CCD(1) which is pooled together with sec 80C for an overall limit of Rs. 1.5 lakhs. So if a person whose limit is exhausted u/s 80C and still wanted to invest in NPS he should not be given an unfair treatment of his investment by not allowing deductions to his gross total income. Hence sec 80CCD(1B) is introduced with an additional cap of Rs. 50,000.
3. In your case, principal repayment cannot be claimed u/s 80CCD(1B) and it is not possible to carry forward the same to the next year.
Please correct me if the above interpretation has an alternative view.