The workaround is to delete the $300 in the error check. Then go back to Deductions & Credits and enter the contribution there, in the Donations to Charity topic. Then click Done with Donations and proceed through the screens until you get to the screen that tells you that TurboTax has added in the contribution.
The contribution will be back on Form 1040 line 10b, but it should not be flagged as an error if you enter it in Donations to Charity.
@jelliswp
