Two things, maxeaus...
(i) The trading terms with the newsletter are 'exclusive' (to them) for 30 days....That means, of course, you are free to negotiate with any other party re the name after that 30 day period is up.
(ii) If the 'buyer' hasn't coffed up after 30 days, then, its probable he isn't going to....Again, I'd say you're free of any obligation to sell to that buyer, after that length of time.
I suggest you consider yourself free to deal in that domain any way you want, as of now.
...But, at the same time, I'd be asking the newsletter what their feedback re the 'buyer' is, as well - and, if the money does materialise, then, you can always sell it to that buyer....But, you're not obliged to now, imo.
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