How do I formulate in a condominium sales contract if I not know the final buyer yet?

You can do that, but better practice may be to wait for the offer to come in. Typically, the buyer's agent will draft the offer, which becomes the sales contract when you accept. In addition, you don't know what is important to the buyer. You may address things they don't care about and give them another bargaining chip.

Never sign an agreement without knowing all of the parties and all of the conditions. If you don't know who will buy your condo, there is no one to bind to the contract, making it worthless.