thanks Wahine for the clarification and suggestion.

I have already read the offer letter but there was 
nothing in the letter which

indicates any penalty for early resignation.  It is 
simply a standard letter of offer

stating the position, salary + allowances , probationary perod etc.

I dont know how to ask HR department to put it in writing and include "Non-penalty" for early resignation.

Any further suggestion or comments?

Thanks