The other respondents have answered your question regarding commissions very well. I would like to point out another important issue, though:
I would make sure the option money you receive is adequate compensation should the transaction crumble--and would do a thorough credit and background check before turning the property over to these tenants.
Commissions paid could be chump change compared to the costs of eviction and property rehabilitation.
I do not know real estate law in your area (disclaimer).
It seems to me that if there is only a four month window, why would you not simply close escrow in four months? Maybe I am misunderstanding your question.
If the question is, they are going to lease the property for four months, and then, at the end of four months, open esrow, from where I work that is still two transactions with the paperwork that goes with them. That's a lot of work.
Remember, Realtors get paid for results, not time. If the Realtor is representing both buyer and seller (dual agency) and you don't think that they deserve the full commission, that's an arguement you need to have to the Realtor. If this is really the best offer is the question. I don't know your market, by looking at the CMA you should know if the offer is fair. Forget the lease part of it for a minute.
So far as the lease commission goes, I would go along with Linttte's point and simply discuss it with your Realtor. I don't know your market. Maybe this is a great offer and the best the Realtor could do in your market.
Just remember, nothing is sold until escrow closes. Most lease options in my experience to do "option". They just lease.