Better have someone proofread it.
Letter to boss, scenario 1: Boss, it seems as if my role in your small company has changed. I know you're in charge, but I don't like what's going on here. I'll give you $20K for a % of interest in your company (this insinuates you would have a say in changes to be made BTW).
Letter to boss, scenario 2: Boss, I have noticed a trend for the positive in our business growth in the recent past, and I wanted to share some thoughts with you. Our sales have increased X% while our profits have increased Y%. By looking at the numbers for the past year, my sales account for _______. In addition, I have trained all current employees whose sales account for __________. I am very happy with the way things have been going, but I wanted to bring something to your attention. You may not know that with some of the increased job responsibilities I have, my potential for the same sales numbers this year may be limited. This may be detrimental for both of us. I would like to discuss with you, in detail, how we might optimize my skills for the growth of the company. In addition, if you would like, I would even be willing to invest $20K in this company for a 15% interest. I feel this is an indicator of my belief in this company, and my dedication to seeing it grow to its full potential. I look forward to your reply.