What could possibly be troubling you? As you can see from many of the answers, you have no obligations, and there are no hard and fast ethical rules that apply other than this one - The Golden Rule:
"He who has the gold makes the rules..."
If it's all been verbal, there's been no agreement. Real estate agreements must be in writing.
Are you going back on your word? If it were some other sort of purchase--a tricycle or a couch--probably so. Such deals can be negotiated verbally and in some cases can even be binding. But real estate? No.
I do have to wonder, though, whether any real estate agents have been involved. You should have been receiving advice from your agent about what can and can't be done.
Advice: Get an agent. Then submit an offer in writing.
If you're buying FSBO, still get an agent. And if you don't want an agent, then have a lawyer assist you.
Hope that helps.
That said you run the high risk of irritating the seller and having them walk away. If you are okay with that, the take your shot. If you really must have the house, try to work with your agent to developed a strategy to make the move more comfortable while retaining the sale.
The realtors code of ethics would not preclude what you are proposing.
Licensed MN Broker
Coldwell Banker Burnet
The second, how far apart was your offer and the counter? I have watched hundreds of people in the new construction arena purchase a basic package then sweat bullets over a $5,000 upgrade. My point is, if the offers were not a great deal apart you probably shouldn’t be buying this house anyway. I can’t imagine with today’s interest rates how much the counter would need to be to make it unaffordable. Did it make more than $100 difference in payments? Nothing magic about a $100, just pointing out reality. We agreed to a $1500 a month payment but backed out when it went to $1525. If you are cutting it that close it may be time to step back.
NMLS # 6395
All the parties just need to come together and understand where you are coming from. If the Seller can't accept your original offer, then move on to the next option. Don't put yourself in a position to be buying more than you are comfortable with/can afford.