Great question! I went from a large firm to a small business and became the manager quickly. I ran and managed the entire office and that is how my experience relates.
There are multiple ways to keep the manager enticed to stay in your organization. Here are a few ideas that have worked, could work, and other ideas that I have heard.
1. Pay - of course providing a higher pay to the manager to retain them. This shows that they are valued to the growth of the company. You are investing in the manager as a key piece to keep the company growing.
2. Providing Education - Paying for classes to help this certain employee grow personally. Ideally, paying for the classes to help this employee grow professionally will help both your company and your employees growth.
3. Retirement - provide a retirement account for the employee if you do not have one yet.
4. Student Loans - help by paying for part of their student loans. Obviously this relates to pay but this is a different way that can keep the employee enticed to stay.
5. Delegate - if they are that good and qualified to keep, allow them for responsibility/freedom to incorporate their ideas so they feel the value they are bringing to the company.
This is just a few example but I hope these help and if you want more ideas, please reach out. good luck!