Hi (Recipient's name),
Thank you for your job offer (insert details). Before accepting, I would like to discuss the salary.
I believe the current salary offer doesn’t reflect my (skills, experience, seniority, achievements, etc.).
Please let me know what the process is. I believe that, between us, we can agree on a starting salary that we are both happy with.
I am available at (insert contact detail and dates and times).
Kind regards,
(Your name)