Al-ʿUtbī related to Yaḥyā from Ibn al-Qāsim that his opinion on this issue varied: at one time he said, “The last portion of the time belongs to the later of the two prayers,” and at another he said, “The last portion is for whichever of the two has been missed.” On this point they display a good deal of inconsistency.
The safest course—and the one most worthy of adoption and reliance—is that if a traveler departs at a time when, because of duties that must take precedence, he cannot pray until the time has elapsed, he should regard that prayer as one whose period has already passed and make it up accordingly.
If a traveler leads a congregation consisting of both travelers and residents, the travelers give salām after two rakʿahs, while the residents stand and complete the prayer on their own, each individually. Should the residents appoint one of those who must complete the prayer to lead them, their prayer is invalid according to Mālik and most scholars.
When travelers and residents gather, it is preferable for a traveler to lead. If a resident leads and the travelers catch at least one rakʿah with him, then all of them must pray in full. Allah alone grants success.