Ramifications of withdrawing publication offer (question from student editor)?
I believe the author is in the right here. Accepting an article is morally binding and should commit the journal to publish it, barring exceptional circumstances. Authors should be entitled to rely on that commitment.

Although you may not have any legal obligation to publish the paper at this point, to refuse to do so would be extremely unprofessional and reflect very poorly on the journal. Given the deadlines, you can't publish in the current issue, but you should queue it for publication in the next issue with available space, ahead of any other articles not yet accepted.

Your idea seems to be that the incoming editor should have sole authority to determine the content  of the next issue, and as such that he should have the right to decide not to publish the article if he so chooses. I would disagree with that. I believe the incoming editor has a  professional duty to treat your acceptance of the article as binding on him as well. If you want to think of the journal as a continuing entity, rather than a sequence of unrelated anthologies, there needs to be continuity of this kind.

It is somewhat awkward for the incoming editor, as now he will have less space to fill with articles he selects. He may have a right to be annoyed with your poor planning, but that's between the two of you; you shouldn't make the author suffer for it.

I'd make exceptions only in very unusual cases, such as if the incoming editor discovers fraud or plagiarism in the article, or finds that your review process grossly violated the journal's stated standards.