A listing agent has a fiduciary duty to the seller. Following her wishes may not be in her best financial interest.  She may never get an offer higher than $558,000.

 I don't know your market or how long it's been on the market but I'm sure you do. The fact that she says she must sell is a good sign that she is a motivated seller.

I just had a very similar situation. When I first went on the listing presentation she wanted $750,000. I thought it would sell for between $650,000 - $700,000.  I told her I would list it at $699,000 and if it was worth over $700,000 through my marketing strategy and exposure the market would bid it up.

She seemed agreeable but said she wanted to think about it. The next day she called to say she was going to list with another agent for three months who told her she could get her $825,000. The agent said she had special buyers.  I asked her if she signed yet she said no. I asked her if I could come see her in the next half hour.

When I got there I joked about the other agent. I said I wish she could get a million but it's not realistic. 

We sat down at her computer and I logged on to my mls  the NYT times yahoo everything online. I showed her sold currently on the market and ones that didn't sell.

 She paid $400,000 2 years ago. The last sale in the building same apartment was $495,000. I convinced her and she listed it with me for $699,000. at 6%.

She called the other agent and said if she brought a buyer for $825,000 she would pay her an extra 1%. 3% to me 4% to the other agent.

I had a comprehensive marketing plan the apartment.  I set up Broker Open House tours in the neighborhood with lunch served courtesy of Wells Fargo mortgage, emailed 4/color flyers to every agent in my market,  Open Houses, advertising, internet strategy bla bla bla.

Only 1 offer in 3 months. $650,000. She didn't want to hear it "Tell them I'm not accepting $650,000. The agent who's buyer offered $650,000  from the minute I met them started telling me how overpriced the listing was and we are in a different market now. I convinced my seller to counter. I said it's always a good idea to start negotiating when its all you have. If a better one comes along I'm still showing. You don't have to sign anything. In my market unfortunately brokers are not allowed to write contracts only lawyers. So we do everything verbal or written but it doesn't mean anything until both buyer and seller sign the contract. So even if a seller accepts an offer they can take a better one until the contract is signed.

The buyers came up to $675,000 but she got stuck at $677,500. We were at a stand still. They were good buyers putting down $200,000. I called the other agent and said we're $2500 apart it's a 6% commission many are 5% would  she split the $2500 with me $1250 each. She agreed the seller agreed we had a deal and it sold and closed for $675,000 a $275,000 profit in 2 years. Where else can you make that kind of money. Take the money and run.  I hope this helps.