Thanks all for the advice. The guy used to work for REMAX in Zug, but since he has left, I think this has nothing to do with REMAX Zug.
I looked the contract again. He send the offer in name of RE/MAX Zug, but in contract he made "xxx(his name) Gmbh, REMAX Zug". We never signed any other contract with his new company. In addition, in contract with REMAX, it also quote selling price and a minimum selling price. For the contract we signed with the buyer, the price is much lower than the minimum selling price we agreed in contract with REMAX.
In contract it shows how much percentage we should pay after selling is completed. But not clear how much we have to pay if terminated, it has a 500CHF, plus cost.

In addition, in the selling contract with buyer, the agent specified that deposit goes to his company account. Since we had no experience before, we thought it is standard, anyway we have to pay him commission after transaction.

I agree it is a good idea to join this owner association, it is a pity that we don't know this before.
Anybody can recommend a good lawyer on this issue in Zug?

thanks!