Allah's statement,

قُل لَّوْ أَنَّ عِندِي مَا تَسْتَعْجِلُونَ بِهِ لَقُضِيَ الأَمْرُ بَيْنِي وَبَيْنَكُمْ ...

Say: "If I had that which you are asking for impatiently (the torment), the matter would have been settled at once between you and I,''

means, if I have what you ask for, I will surely send down what you deserve of it,

... وَاللّهُ أَعْلَمُ بِالظَّالِمِينَ ﴿٥٨﴾

but Allah knows best the wrongdoers.

Someone might ask about the meaning of this Ayah compared to the Hadith in the Two Sahihs, from Aishah, may Allah be pleased with her, that;

she said to the Messenger, "O Allah's Messenger ! Have you encountered a day harder than the day (of the battle) of Uhud?''

The Prophet replied,

لَقَدْ لَقِيتُ مِنْ قَوْمِكِ، وَكَانَ أَشَدَّ مَا لَقِيتُ مِنْهُمْ يَوْمَ الْعَقَبَةِ،إِذْ عَرَضْتُ نَفْسِي عَلَى ابْنِ عَبْدِيَالِيلَ بْنِ عَبْدِكُلَالٍ، فَلَمْ يُجِبْنِي إِلَى مَا أَرَدْتُ، فَانْطَلَقْتُ وَأَنَا مَهْمُومٌ عَلَى وَجْهِي، فَلَمْ أَسْتَفِقْ إِلَّا بِقَرْنِ الثَّعَالِبِ، فَرَفَعْتُ رَأْسِي، فَإِذَا أَنَا بِسَحَاَبَةٍ قَدْ ظَلَّلَتْنِي،

Your people have troubled me a lot and the worst trouble was on the day of Aqabah when I presented myself to Ibn Abd Yalil bin Abd Kulal, who did not respond to my call. So I departed, overwhelmed with severe sorrow, proceeded on and could not relax until I found myself at Qarn Ath-Tha`alib where I raised my head towards the sky to see a cloud unexpectedly shading me.

فَنَظَرْتُ فَإِذَا فِيهَا جِبْرِيلُ عَلَيْهِ السَّلَامُ، فَنَادَانِي فَقَالَ: إِنَّ الله قَدْ سَمِعَ قَوْلَ قَوْمِكَ لَكَ، وَمَا رَدُّوا عَلَيْكَ، وَقَدْ بَعَثَ إِلَيْكَ مَلَكَ الْجِبَالِ، لِتَأْمُرَهُ بِمَا شِئْتَ فِيهِمْ،

I looked up and saw Jibril in it and he called me saying, `Indeed Allah has heard what you said to the people and what they have responded to you. Therefore, Allah has sent the Angel of the Mountains to you so that you may order him to do whatever you wish to these people.'

فَنَادَانِي مَلَكُ الْجِبَالِ وَسَلَّمَ عَلَيَّ، ثُمَّ قَالَ: يَا مُحَمَّدُ إِنَّ اللهَ قَدْ سَمِعَ قَوْلَ قَوْمِكَ لَكَ، وَقَدْ بَعَثَنِي رَبُّكَ إِلَيْكَ، لِتَأْمُرَنِي بِأَمْرِكَ فِيمَا شِئْتَ، إِنْ شِئْتَ أَطْبَقْتُ عَلَيْهِمُ الْأَخْشَبَيْن

The Angel of the Mountains called and greeted me, and then said, `O Muhammad! verily, Allah has heard how your people responded to you and He has sent me to you so that you could order me to do what you wish. If you like, I will let Al-Akhshabayn (two mountains to the north and south of Makkah) fall on them.'

فَقَالَ رَسُولَُ اللهَ

بَلْ أَرْجُو أَنْ يُخْرِجَ اللهُ مِنْ أَصْلَابِهِمْ، مَنْ يَعْبُدُ اللهَ لَا يُشْرِكُ بِهِ شَيْئًا

The Prophet said, No, but I hope that Allah will let them generate offspring who will worship Allah Alone, and will worship none besides Him.

This is the wording of Muslim.

Tormenting the disbelievers of Quraysh was offered to the Prophet, but he chose patience and asked Allah for respite for them, so that Allah might let them generate offspring who will not associate anything with Him in worship.

Therefore, how can we combine the meaning of this Hadith and the honorable Ayah,

قُل لَّوْ أَنَّ عِندِي مَا تَسْتَعْجِلُونَ بِهِ لَقُضِيَ الأَمْرُ بَيْنِي وَبَيْنَكُمْ وَاللّهُ أَعْلَمُ بِالظَّالِمِينَ

Say: "If I had that which you are asking for impatiently (the torment), the matter would have been settled at once between you and I, but Allah knows best the wrongdoers.''

The answer to this question is, Allah knows the best, that the Ayah states that if the punishment that they asked for was in the Prophet's hand at the time, he would have sent it on them as they asked.

As for the Hadith, the disbelievers did not ask the Prophet to send the torment down on them. Rather, the angel responsible for the mountains offered him the choice to let the two mountains to the north and south of Makkah close in on the disbelievers and crush them. The Prophet did not wish that and asked for respite out of compassion for them.