If both sides signed, then I would talk with my lawyer and see what they have to say.

But having your license can sometimes be a bad thing if (and more likely, when) a deal falls apart long after you assigned, and the seller/buyer get "lawyered" up to try to get the assignment fee that they paid you a long time ago (even though your one and only obligation in the deal was to assign your option to them, and you have no further liability/obligation after you assign.

I suggest you get your own lawyer.  

(This can be risky; use a lawyer.)And everything is negotiable: price, interest rate, who maintains what, time periods, everything.

I have my broker, but she has some commercial experience, but mostly residential.Is it advised to get a lawyer involved and CPA to analyze the books, etc?

Not a Lawyer, but i think you just did it, just put in the sales agreement that the spot already has a renter and that they are to agree, and sign the agreement, that if they buy it that they will honor the current renters lease as a contingency for buying it, and provide them with the rental agreement.

Oh, and this month we  are having a one of the owners/ lawyer of Common Wealth Trust Services speak about setting up Land Trusts to do your real estate investing.Have a blessed Day!

Once in a while there will be a lawyer involved who wants to make changes in the note.

I am not a lawyer or regulator.

What do you need a lawyer for?