Ok you were put under pressure on a deadline mode that created urgency. Which leads to a panic of getting out of the situation and accept because you are uncomfortable.
Go back to the negotiation table. Ask this, I am sorry I was caught a bit offguard and might have misinterpreted our salary combersation. Do you think given ‘ put some successes that got you the promotion’ to the company is only worth $5K? The answer you will probably get is ‘no’ which is where you begin. Ask what do we do about that. And pause, let him fill the silence. Mirror back the last three four words he said, pause, he will elaborate, he will feel the need to… from there, it is your opening. Dont quote a price yourself first. Wait for his anchor. And then say that is a good start, how about x, would that be worth us to achieve x,y,z- anchor it at s number that is a bit higher than what you want, and then see what happens:) good luck!!