Is is a question of ethics?

My customer made an offer on a property. The listing agent presented it - we didn't come to agreement but we were close. My buyer said no deal so we dropped it. The listing agent calls back about two weeks later and said, if your buyer is still interested, we'll take his offer after all. So I called my buyer, he said yes so we brought the offer to the listing agent - with a three day deadline for signatures. I get a call two days later from the listing agent. "just wanted to let you know we got another offer on the property and I have to present that offer with yours." Hmmm... he told me over the phone we had a deal. I know, no signatures no deal... but a man's word isn't what it once was. He presented the offer from his buyer to his seller. No surprise, we didn't come out on top. Had I been in this situation, I would have accepted the new offer as a back-up in case our deal fell apart but a man's word should be worth something. Said he had to watch out for his client's best interests. I now have a customer who doesn't want to look at anymore properties because he cannot trust real estate agents. Note: I was very careful to explain that we were outbid and that's how we lost the deal. I did not bad-mouth or speak lowly of that agent. I did explain that since out deal wasn't signed, he had to present that offer. That's really a gray area for me though. I feel like when you tell someone you have a deal, you have a deal. What are your thoughts on this? Plesae tell me how you would've dealt with this situation if you were on either side of this. Still learning every day...