A refund, regardless of the amount doesn't figure into the Leaf $7,500 at all.
A refund means you gave the IRS too much money throughout the year. If you gave them $100, and your tax bill was $25 for the year, then you get a $75 refund because that is the amount you overpaid. If you gave the IRS $15,000 throughout the year and you owe $12,500 in taxes, you would get a refund of $2,500. What you need to know is what is your tax liability for the year??
Do you pay $10 in taxes or $10,000 in taxes? THAT is where the $7,500 tax credit for the Leaf comes in.
If you only owe $2,000 in taxes, then when you subtract the $7,500 Leaf tax credit, you would get credit for the $2,000 you normally would owe, and you would LOSE THE OTHER $5,500 FROM THE LEAF CREDIT. It is only a credit, not a rebate....there is a difference. You only get the full $7,500 if you pay over $7,500 in taxes.
If, however, you owe $10,000 in taxes, you subtract from that the $7,500 from the Leaf and now you only owe $2,500 for the year. If you had $12,000 withheld throughout the year, your refund would be $9,500 ($7,500 from the Leaf credit, and $2,000 from what you overpaid)
Your refund is simply predicated by how much you have withheld vs how much you owe. The Leaf $7,500 is applied only against how much you OWE, and only UP TO $7,500. If you LEASE a Leaf, however, Nissan 'gives' you credit for the entire $7,500 as if you put that in as a down payment. So if you owe less than $7,500 in taxes, leasing is a way of getting all of the $7,500 that you otherwise wouldn't have.
Clear as mud?