AL-Khazraji, in his book نفس الصباح في غريب القرآن وناسخه ومنسوخه, volume 1, page 376, states that Qataada made this abrogation claim and that the majority agrees. Ibn Hazm Al-Andalusi too.
Professor Al-Mi`yaar Al-Idreesi, who presented the book, notes that scholars have not fully quoted the abrogating verse, so it could be
Which is what Qataada, Ikrima, Mujaahid, Al-Hasan and An-Nahhaas stated. Those who have not been specific include Al-Khazraji, Makki, Ibn Salaama and Ibn Al-Jawzi.
The context of both verses is defense alliances. IMHO, there is no abrogation because the two verses speak about two different sets of conditions for rushing to aid fellow Muslims:
- Muslims who migrated to Medina and those who fostered and helped them must always come to each other's aid in battle. That's what 8:75 speaks about.
- Muslims who did not migrate to Medina may only be aided in battle if two conditions are met:
That is what 8:68 is speaking about.
- They ask for help, and
- Muslims do not have a peace treaty with the enemies of the non-migrating Muslims asking for help. The peace treaty trumps the obligation to help fellow Muslims in this case! Talk about honoring your word and keeping a promise!
Thus, the two verses together make up this one consistent ruling,"Muslims must rush to help fellow Muslims who have been attacked by enemies, unless the helpers have a peace treaty with the enemies of the fellow Muslims asking for help but have chosen to remain in Mecca and not join the prophet (PBUH) by migrating to Medina."
Why is that? Because God wanted all Muslims who can to migrate to Medina. In Medina, they would be protected physically and spiritually and would learn the many new laws that are revealed there. Choosing to stay behind in Mecca risks losing all that and there is no worldly reason that can make up for that. God has made this point clear in several verses, for example,