Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [Hiring Manager],
I am pleased by your offer of a [position title] position with [Company]. This position, and your company, are very suited to my abilities and career path. Likewise, I feel that I can have a positive impact on your profitability.
I am certain that we can come to a mutually beneficial employment agreement. My interview was very informative and gave me a clear picture of how I will fit into and contribute to your organization. As I believe in my abilities in this position, I humbly ask that we renegotiate the [salary/terms] of your offer. I would be delighted to accept an offer which includes [new terms]. This is a reasonable figure and I know that you will see my job performance as an asset.
I will call you to discuss this matter further. I look forward to speaking with you.