1) you have not executed any undertaking to serve bank for period of 5 years . hence you cannot be called upon to pay Rs 25000 as per the undertaking
2)you are liable to refund only 50 per cent of basic salary received by you
3) Base salary - pay before any commissions, bonus, etc.
Gross salary - total pay before any taxes
Net salary - take home pay, after taxes
4) you are not liable to pay 50 per cent of net salary