Is there a sin incurred by one who issued a fatwa allowing the offering of the Isha prayer before the Maghrib prayer when combining them due to a car breakdown and delay, knowing that the mufti was not certain of the fatwa at the time?

Giving a fatwa (definition) without knowledge is sinful, even if one happens to be correct. If someone misses the Maghrib prayer until the time for Isha prayer begins, they should pray Maghrib first, then Isha. This is because there is a difference of opinion among jurists regarding the obligation of sequence (tartib), but the majority of them have stated its obligation between missed prayers and the current prayer. One should prioritize the missed prayers in their due sequence, unless the time for the current prayer is very short, in which case one should begin with it. As for the Shafi'is, they hold that sequencing is desirable (mustahabb) but not obligatory. There is no difference in the legality of sequencing between a resident and a traveler.