Here is a sample acknowledgment letter for a manager.
This is to acknowledge that [Name] is the manager of the [Department] at [Company]. He/She has been holding this position of authority since [Date] with our trust.
The person in question is a reliable and accomplished manager and heads operations smoothly.
This is to acknowledge the assistance and guidance provided to me/us by [You]/[Name], as the manager of [Department and Company]. I/We would also like to extend my/our gratitude for the same.