I understood this sentence as he is our buyer, that is the person responsible for the company purchases.
Did not think for a second it could mean customer!
Buyer vs Client. A buyer is often someone who's job it is to buy (stuff for a business). A buyer would also be someone ready to buy your old car or house.
Tried "He is definitely our buyer" -- rejected 2021-2-15. Looking at comments, I'd say the best options is "actually" , and the presumed context would be a response to a question like "who is that guy?"
Also-- can Kaeufer" be either a customer (who has bought something) or an employee who buys merchandise for a firm to sell to customers?