Other answers say “no” and I initially thought the same, but I’m going to suggest a slightly different approach.
There is a way you could (note the emphasis on the conditional) get out of this with $50,000 (what you seem to be wanting now), but it will take more than just “oh sorry I changed my mind”. If you do that you’ll be crammed, and rightly so.
Basically, your only way forward at this point is to wait for a few days and let this company know that you really, crazily want to work with them (and explain why), however you just received an offer from another company for $52,000. Obviously you thinking about it because $7,000 is quite a difference. However, if they’re willing to bump you to $50,000 it would then be a case closed and you guarantee them that you will join and don’t do any further discussions, and you’ll be extremely grateful for their patience.
Also, if and when you do that, explicitly mention that you’re embarrassed and aware of the fact that you’re borderline, and also insist that you thank them for having already agreed for $45,000. Phrase it like you’re not playing a numbers game, and the only reason for this behavior is that you can’t easily let go of a 7,000 difference because you have responsibilities and all the likes. Your family wouldn’t be happy for instance, or you have student debt, etc.
I’m not saying that it will work. It very well might not work and still cram you, depending on how you did during the interviews. I’m just saying that this is the only sensible scenario in which it might have a slight chance to work.
Be aware that if they go along with that, you’d better perform exceptionally well once on the job, or else you’ll be dumped at the first review (and very likely before that).