Is a man's marriage to a female teacher, despite the objection of his first wife, whom he previously stipulated not to work, considered a breach of the condition of justice? And is his allowing her to work now also considered as such?

That would not constitute a breach of the condition of justice, and there is no blame upon him if he permits his first wife to work. However, he should consider the family's affairs, the household's interests, and the permissibility of each wife's work.