Until you cover that short, you don't pay anything extra. When you cover, you gain (or lose) the difference, subtract any commissions. The brokerage doesn't gain on any price change in between.
However, you should note that, since you're trading on margin, if your maintenance excess becomes undermargined (if you use too much, and your usable margin goes negative), the brokerage can issue a margin call, which is essentially a demand for you to close your position(s) ASAP (usually immediately, or at most a few days) to make up for the negative balance, or deposit more funds to cover that balance.
Note also that, if you are short selling, each day, the brokerage will adjust your margin, depending on how your positions changed in value. That is to say, if your short position turns out to go up by $1, you would lose some maintenance excess to use. This is because the brokerage adjusts it in case the position flops, so you don't use too much margin (their money). Inversely, if your position is gaining, they give you more maintenance excess, since it's a gaining position, and there's less worry that it'll flop.
Maintenance excess is a way for the brokerage to keep a check on your short positions, to prevent you from overborrowing, in case your trades fail.
You never pay the broker for the difference. When you sell short you borrow stock, you repay that same stock; the broker is strictly an agent.
What your broker will do is monitor the transaction to ensure you do not incur a loss greater than your account's ability to buy the shares back. This is done with journal entries. If your unrealized loss nears the point where your ability to buy it back comes into doubt, the broker will cover the short.
In case you didn't read the margin agreement: when you are short selling, your broker has the right to cover that short at any time. With or without notifying you first.
You may or may not encounter a margin fee. But that will depend on everything happening in your account, not any one specific transaction.
short sell of the stock price $10.
Day 1: stock went up $1, then in the afternoon went down $2. (went up 1 dollar to 11 dollars then 2 dollars down becomes 9 dollars)
day 2: stock went down another 2 dollar, then went up $1. (2 dollars down to 7 then up 1 dollar to 8)
do I have to pay the difference to broker whenever stock goes up when i am shortselling?
for example, for one week stock went up down, at the end stock went down, all i have to pay the borrowing fee, is it true?
