If it was simple, lawyers wouldnt be in business. Real estate law especially dealing with a city should not be taken lightly. A realtor would be able to complete it for 1/2% to 1% but even then , a lawyers review may be worth while.
A counter offer needs to br written the same way a purchase and sales agreement would be written. It is bets to be on an official standard Purchase and sales that all real estateagents use as it will contains protections for you as well as any dsiclosures (both state and federal) which may still be needed even though selling to a city.
An attorney will charge by the hour and should give yo a free estimate. Saving pennies to lose dollars doesnt make sense. Make sure you do this right... more