I found it very odd too that he wants to purchase it through Telegram's official username auction platform. Telegram is the only platform that doesn't consider username selling a ToS violation and they even have their own auction platform but I've never been asked to actually do a transaction through he 'official' means even though it prevents the possibility of scamming.
Maybe this guy isn't overly familiar with how this usually works. The value of the username is roughly $20k so even for some obscenely rich buyer, making a starting offer of $300k is basically throwing away over $250k.
$50k would've been completely ridiculous as well but I probably would've responded because it's not entirely unheard of for wealthy buyers to make an offer so far outside of its value that no one would refuse it.
Specifically Middle Eastern collectors tend to do this so him owning a lot of UAE and other Middle East related usernames checks out in that department.
I think I've mentioned it before at some point but an old transaction with a wealthy Saudi always stuck with me. The guy said that at his level of wealth, everyone can buy Lamborghinis, Bugattis, exotic animals, ... but only one person can own a specific username.
A lot of 1 character usernames on IG for example are banned at this point because buying usernames is technically a ToS violation, in addition to often being targeted by hackers/SIM-swappers, but at one point the alphabet used to be almost entirely owned by Middle Eastern buyers like this guy.