Is this covenant binding on the Muslim for life, despite Christianity's breach of the covenant's condition, or is he relieved of the obligation and nothing is incumbent upon him? And if there is an expiation, what is it?

Fulfilling a covenant is a religious obligation. However, if the covenant is conditional, then it is not obligatory to fulfill it if the condition is not met. Therefore, this person is not bound by this covenant because it was conditioned on the woman embracing Islam within three months, which did not occur. Thus, the covenant becomes void. It is important to note that if a woman embraces Islam and it is ruled that she must separate from her disbelieving husband, she must observe an 'iddah (waiting period) before marrying another man. Her husband has a greater right to her if he embraces Islam while she is still in her 'iddah period.