He was prepared to give more than any one else.

Nobody would go above his figure, he had set it so high--higher even than Rocket was really worth.
Five hundred and fifty, if necessary.

No one would rise above that, Harney was sure, and quietly waited until the bids were far between, and the auctioneer still dwelling upon the last, seemed waiting expectantly for something.
"I believe my soul the fellow knows I mean to have that horse," thought Harney, and with an air which said, "that settles it," he called out in loud, clear tones, "Four hundred," thus adding fifty at one bid.
There was a slight movement then in the upper balcony, an opening of the glass door, and a suppressed whisper ran through the crowd, as Alice came out and stood by the colonel's aide.
The bidding went on briskly now, each bidder raising a few dollars, till four hundred and fifty was reached, and then there came a pause, broken only by the voice of the excited Claib, who, as he confessed to Hugh, had ventured to speak for himself, and was rewarded for his temerity by a blow from Harney.

With that blow still tingling about his ears and confusing his senses, Claib could not well tell whence or from whom came that silvery, half-tremulous voice, which passed so like an electric shock through the eager crowd, and rousing Harney to a perfect fury.
"Five hundred." There was no mistaking the words, and with a muttered curse at the fair bidder shrinking behind the colonel, and blushing, as if in shame, Harney yelled out his big price, all he had meant to give.

He was mad with rage, for he knew well for whom that fair Northern girl was interested.