My employer's initial job offer list the following terms for termination, this agreement was signed before i joined the company here in SG :

However the detailed employment agreement sent to me and signed after i arrived here list different termination clauses

the clause of having to pay for one months salary is missing from/ not present in the employment agreement
My company HR is now stating that they will not pay one month salary as i am under 1 year , however i am not agreeing to it as my employment agreement does not list the clause .
I am contesting that the terms in employment agreement are applicable as it was signed after the initial job offer. However my company HR is refuting and stating the job offer terms supersede employment agreement
Am i correct in my refusal and can i approach MOM for arbitration for this dispute ?