Suffolk Downs announced early Friday that a $5 million bonus will be offered for the Sept. 20 Massachusetts Handicap if both Big Brown and Curlin run with undefeated records in 2008. The bonus would include a guaranteed $1 million bonus for each horse. The purse for the 1 1/8-mile stakes at the East Boston, Mass., track currently is $500,000. A $3 million bonus, including $500,000 for each horse participating, would be offered if Big Brown wins the Triple Crown and both he and Curlin run, but either or both of them have lost a race this year. The bonus is being offered by Coastal Development, a company controlled by Richard T. Fields, the largest shareholder at Suffolk.