Dear Sangeeta, Every company has management guidelines in the format of a "Schedule of Power" (SOP). As per this SOP, management assigns power to its executives. In your case, obtain a copy of the SOP of the organization. Check to see whether the HR manager is an authorized signatory for offer letters. If not, then find out who the authorized signatory is and ask for a Power of Attorney.
In this Power of Attorney letter, it will be mentioned that the HR manager will be authorized to sign offer letters on behalf of the authorized person as stated in the SOP. This letter will be signed by the authorized person, yourself (for accepting), and the HR Head. It will be helpful in your audit as well.
I hope this clears your doubt.
Thanks & Regards,
Anupam Anurag.