You own the domain name, legitimately, so you are not "giving it back" you are "selling it", and back to the seller for the same price means only they win out!
It's yours to sell now, so if someone else wants it, albeit through the seller, then you are entitled to put your price on it, even if just a split of the new sale profits.
If the seller is not just wanting to get as much cash as possible, and are just concerned about the mistake they made (we all make 'em), then they should have no issues with splitting the profit of the sale to the other buyer. The percentage should be 50% at least as you own the domain and are selling it, and they are just rectifying a mistake - and
still making more money. If they haggle at all or too much then they're perhaps just being greedy.
If I was that seller who made a mistake and saw you were even thinking about trying to sort this out I'd be willing to give you 100% profit from the resale if this was only about sorting a problem! If it's about making more money then that's their issue and they should not have sold to you!
Look, if you bought a car from a dealership, and on your way out the manager comes up and asks you to give the car back and you will get a refund, as someone else has offered more for it. Your response is...?
Of course it is, second word is "off"...
And a domain name sale is different because?
Without more info (the sellers attitude etc) it's hard to give an accurate response as to what I would do personally. With the info I have, I'd probs allow them to make the sale on Godaddy so they keep face on Godaddy and not cause issues etc, and take a percentage that pays for time dealing with research, the purchase, etc. So a quick flip.
And of course, Escrow all the way. (I guess they would handle the sale in Godaddy, but give your Escrow details, you transfer, get the money, and give them the agreed amount - you are already trustable as you paid for it once
)
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All that said, the domain name is yours and entirely fair and square, and what you do with it (give back, keep, sell and split profits) is
entirely up to you and carries no negative moral implications.