I sent out a simultaneous submission and got an offer from one publication almost instantly. I agreed to their terms and agreed to write the article. The very next day, a higher paying publication said they wanted to buy it. Can I now decline the first one so I can get more money from the second publication?
If you agreed in writing or even verbally to do the work for the first publication, you are bound to that agreement. While you can probably get out of the contract (I doubt they’d sue you over something as silly as backing out of a deal), you shouldn’t expect any future work from the publisher you stiffed. Ethically, you should sell the contract to the first one who makes an offer provided, of course, that you’ve accepted their offer.