Welcome to GMAT Club.
Congrats on the offer and scholarship.
Schools rarely negotiate scholarships as money is usually pre-allocated and usually the only sweetener they throw in is a small fellowship or GA position. I am surprised the student mentioned you could. Perhaps you should ask her/him about how to approach this situation if this has been done.
It is not unheard of for a school to offer something slightly better (e.g. 60k instead of 50k but that's rare and you usually have to have another offer at hand).
I don't know how your position to negotiate would be without another offer in hand and already having paid the deposit. You probably don't want to bluff and say you have an offer (that would be lying and dishonest). The only other way I can think of is to say something has changed and hope the adcom would feel sorry enough to give you more money. In other words probably 5% get more money based on a very non scientific estimate. You are not the one who is missing out really. These opportunities are really for those 5% of us he situations.
Good luck!
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