I have been independently working on a substantial idea. The results of my work have been compiled into a complete manuscript.

However, I think I need to have an expert in the field reviewing it to check for potential flaws prior to publication. Then I want to include him in co-authorship. The expert I look for is a professor in another country and we have had no prior contact.

Is it inappropriate to send him a request via email and explain the aforementioned story?

PS: I am a Master's graduate with previous publications. Any elaborate advice or template on how to approach is much appreciated.