Actually, this is not a tax "credit" (which is offset dollar for dollar against your tax), but is a subtraction from your Maryland taxable income.
In your Maryland interview, you will come to a screen with the heading "Here's the income that Maryland handles differently". Scroll down the page to the subheading "Miscellaneous" and click on Start for "Unreimbursed Expenses of Organ Donation". Enter only your out-of-pocket expenses related to the organ donation that were not reimbursed by your insurance or any other source.
Limit the amount you enter to no more than $7,500. This amount will appear on form 502SU, line ss.
