how does the broker deal with leveraged trades?

Using leverage, the investor has a loan provided by the broker.

To trade a standard lot of $100,000, with a margin of 1%, an investor will have to deposit $1,000 into his or her margin account. The leverage provided on a trade like this is 100:1.

When the investor has a winning trade, he/she is paying the loan.

But what happens when the investor makes a bad trade and get a margin call?
This investor now owes $99,000. How is the loan paid? How does the forex market or brokers deal with this?

the loan paid by leveage

Doesn't work that way. A margin call will come way sooner than when you lose your entire position value. The company I trade with will trigger a margin call at half of margin used (not position value). In your example, when you were at $500 unrealized loss, you would get a margin call and the trade would be closed. You would lose half your account. This ensures that you will have enough money in your account to cover any losses. That is the danger of trading fully leveraged. 50 pip loss and you're out.

The investor still cannot pay the losses. I imagine the broker has to buy or sell in the market the $100,000 for his trade, or not?