All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon all His family and companions.
While combining prayers, the person travelling for a distance of (81KM) and more is allowed to make either taqdim (advancing) or ta’khir (delaying) after crossing the boundaries of his city of residence. Al-Imam Al-Shirbini (May Allah have mercy on him) said, " You are allowed to advance the second prayer to the time of the first prayer. Thus, if you are combining Zuhr and `Asr , you can first pray Zuhr at the time of Zuhr, and then advance `Asr by praying immediately, or if you wish you can defer praying Zuhr until the time of `Asr arrives, in which case, you will first pray Zuhr and then pray `Asr afterwards. The same procedure applies to combining Maghrib and `Isha as well. This is based on the practice of the Prophet (PBUH).
As for making Ta`khir, it is recommended that the Musaafer (Traveler) offers the two prayers in sequence. In other words, he prays the first then the second immediately. However, if he performed the second prayer first, then it is valid.
According to the book {Bushra Al-Kareem, 377-378}, the four conditions for advancing prayer are:
First: Performing the first prayer first since it isits prescribed time and the second prayer follows it, and what comes second doesn`t precede what comes first.
Second: Making the intention of combination.
Third: Performing the second immediately after the first.
Fourth: Continuity of travel until beginning the second prayer.
Moreover, none of the aforementioned conditions is stipulated in Jam` T`akhir {Delayed combination} of prayers since the second prayer follows the First. Rather, two conditions are stipulated for delaying the combination of prayers and they are: First: Making the intent to offer the first prayer within the prescribed time of the second, or else one incurs sin and then must make up for the first. Second: Continuity of travel until finishing the second prayer. And Allah knows best.