Can a person teach a language without yet dominate it?

Personally, I'd rather learn the items you mentioned on my own, but what you describe is pretty common in language exchange so there is a precedence. 

In my opinion teaching definitely helps to cement your knowledge of the language and organize it better in your head. Being in front of a student you are faced with a ton of questions, which force you to rationalize the intricacies of the language, whether through the lens of traditional grammar or through your own study methods, summarize that knowledge, categorize it and teach it in such a way that a beginner can make use of it. In that sense, teaching will almost certainly be beneficial to you. I’ve done it with English in the past and I can attest to it.

Now, the question is whether it is beneficial to the student or not. And that can get particularly dubious if you decide to charge them for that service. Personally I wouldn’t ever charge anyone before I reach at least a solid B2 level in all 4 skills (reading, writing, hearing and speaking), and even then I would adjust my fees accordingly. If that weren’t the case I would do it free of charge.

Having said that, I believe we live in a free market, so people should be able to make their own minds and choose to validate a service with their money if they see it fit. What’s key here is transparency, so both parties have to be aware of what they are getting. In that sense, you should make it perfectly clear that you are an advanced student who has not yet mastered the language but can offer some valuable insight.

I completely agree Marcos. I will be doing my best to teach in a structured way so they can make the most of it. My learning process will probably come from both preparing the lessons and realizing how much I know when teaching. I have known these friends for almost a year and they are amazing at giving me feedback and at being interested in stuff, so the sessions are probably going to be very fun and beneficial for all of us. If they turn out good enough, I will probably be posting my experience/findings through the process.

Also, as you said, I am not charging anything because it would be unfair. I am not an expert, they are aware of that as I clarified it, I also often tell them how my studies are going. It would probably be more accurate to call it a “study session” --or something along those lines-- in which I take a guiding role.