Continue through the rest of the interview. When you get to the Smart Check at the end of the federal interview, it will flag the $300 contribution as an error and tell you that it should be blank. Delete the $300 on the Smart Check error screen and click Continue. The problem will be solved.
Apparently this happens if you enter the $300 deduction before you have entered all your itemized deductions, then you go back and enter enough itemized deductions to exceed the standard deduction. I don't think it would happen if you went through the interview strictly in sequence, without going back to an earlier section.
