I don't think you can claim the full 26.375%. IRS Pub 514 says:
Foreign tax refund.
You cannot take a foreign tax credit for income taxes paid to a foreign country if it is reasonably certain the amount would be refunded, credited, rebated, abated, or forgiven if you made a claim.
To maximize refund, I believe you also have to file a Germany tax return. In the future, it might be possible to adjust withhold settings so that you are not overwithheld.
Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or tax advice.
