Sebi on Wednesday asked stock exchanges to impose heavy penalty on brokers allowing their clients to trade in derivative market without sufficient margin money and said that fines could be as high as the shortfall of funds. While the minimum penalty is 0.5% of the shortfall of margin money, the penalty could be as high as 100%, the Sebi said in a circular. The stock exchanges will have to impose the penalties from September 1, Sebi said.