You're completely mixing up the roles of a broker and a fund manager there. A broker is (usually) just a guy on the phone who you call up and say 'buy me this at this', he has a trader execute the trade, and takes a commission. They can and do offer clients advice, but they will not take a trade on a client's behalf without their express order to do so. A fund manager is someone you give your money to, they trade it on your behalf subject to a performance fee/profit share.
If a MM wants to be short, I don't know, Microsoft, he will let people know via various channels, then the following conversation occurs:
'John, I see you're showing 25,000 shares MSFT at blah blah... I'm a buyer of the 25,000, with another 100,000 behind to work.'
The MM is consequently now short 25,000 shares ie he has put up risk capital on behalf of his client; he must try to offload for as little loss as possible (although sometimes if he's lucky he will be able to profit) - he then has the other 100k to 'work', meaning that if he can buy back the first 25k shares without taking a loss, he can earn the spread on the remaining 100k, with no risk to himself.
You have completely tangled up all your knowledge my friend.