It looks like NYU is unaffordable. I am assuming most, if not all of your NYU FA package is need based aid. If so, I do not think NYU will stack outside scholarships, should you receive any (and it’s really late for many scholarships, especially to the tune of $5k). Look here, and best to contact them: Scholarships and Grants
Now that NYU meets full need, there must be something in CSS Profile that resulted in them calculating a $15k family contribution. Even though you say your FAFSA EFC is zero, what matters to NYU is the info in CSS. I encourage you to understand what factor(s) explain that difference…typical ones would be divorced parents, parents own a business or real estate that’s not a primary home.