Dear Felix
I am delighted by your offer to join [FIRM, ex. XYZ Inc]. I find the position and job duties to be challenging, yet tailored to my abilities.
I am quite pleased with [STATE TERMS WITH WHICH YOU ARE PLEASED, ex. the salary and benefits package.] I humbly request that [STATE REQUEST, ex. we change my starting date from June 15 to June 30 to accommodate engagements I have previously made.] I trust this to be reasonable.
Allow me to call you on [DATE, ex. Thursday] to finalize the offer. I look forward to working at [FIRM, ex. XYZ Inc].
Regards
Lydia