The free conversion requires 2 of the same type of special log, which are also accepted by the sawmill as bank notes. 2 special mahogany logs can be used to convert up to 27 teak logs and/or mahogany logs into teak/mahogany planks saving you up to 40500 coins (27 mahogany logs at 1,500 coins each). If you have 1 special or 1 of each, the sawmill owner will only offer to buy your special logs.