Do not confuse withholding amounts with your tax obligation. The amount your employer withholds is an approximation (a variation on estimated taxes) derived from IRS tables: if you have X pay periods per year for Y dollars each, your total tax due will be Z. That will come pretty close if all of your pay is identical, if you have no other income, if your filing status doesn't change, if you don't qualify for any tax credits, if, if, if.... In other words, again: it's an approximation.
That's why you typically get a refund or owe additional taxes when you file. If you have a total tax obligation of $10,000, and your employer withheld $12,000, you get a $2,000 refund--but your total tax obligation is still $10,000 (you paid it all in advance, plus $2K over, via withholding). If your employer withheld $8,000, you will have to write a check for the $2,000 when you file--but your total tax obligation is still $10,000 (of which you've already paid $8,000 via withholding).
For a non-refundable credit of $7,500 (which is what I understand this to be), it's your total tax obligation that matters: the credit will reduce that total, but not below $0 (some credits are "refundable," meaning if the credit exceeds your total tax obligation you can get money "back" that you never actually paid in). How much is withheld (or paid via estimated taxes or prior years' refunds or any other method) is not relevant.