Learn Russian: Мы обсудили деловое партнёрство с одним иностранным инвестором. - We discussed a business partnership with a certain foreign investor.

In Russian, один is not always just the number one. Here it acts as an indefinite pronoun meaning 'a certain' or 'some particular'. It shows that you have a specific foreign investor in mind, but you either are not naming them or their identity does not matter to the listener. If you dropped одним, it would simply mean 'with a foreign investor' in a general, non-specific sense.

Why is один in the form одним?

It has to agree in gender, number, and case with the noun it modifies, exactly like an adjective. Because the preposition с (meaning 'with') requires the instrumental case, the masculine noun инвестор becomes инвестором. Therefore, один takes its masculine instrumental form, одним.

Why use the perfective обсудили instead of the imperfective обсуждали?

The perfective обсудили emphasizes the completion and result of the action. It implies the meeting happened, the topic was talked through, and the discussion is now finished. If you used обсуждали, it would focus purely on the process, meaning 'we were discussing' or 'we spent time discussing'.

What is the root of the adjective деловое?