وَإِذْ تَأَذَّنَ رَبُّكُمْ

And (remember) when your Lord proclaimed,

means, proclaimed and made known His promise to you.

It is possible that this Ayah means, your Lord has vowed and sworn by His might, grace and exaltness.

Allah said in a similar Ayah,

وَإِذْ تَأَذَّنَ رَبُّكَ لَيَبْعَثَنَّ عَلَيْهِمْ إِلَى يَوْمِ الْقِيَـمَةِ

And (remember) when your Lord declared that He would certainly keep on sending against them (i.e. the Jews), till the Day of Resurrection. (7:167)

Allah said,
 
لَيِن شَكَرْتُمْ لَازِيدَنَّكُمْ

If you give thanks, I will give you more;

meaning, `if you appreciate My favor on you, I will give you more of it,
 
وَلَيِن كَفَرْتُمْ

but if you are thankless,

if you are not thankful for My favors, covering and denying, then,
 
إِنَّ عَذَابِي لَشَدِيدٌ
 
 

verily, My punishment is indeed severe,

by depriving you of the favor and punishing you for being unappreciative of it.'

A Hadith states that,

إِنَّ الْعَبْدَ لَيُحْرَمُ الرِّزْقَ بِالذَّنْبِ يُصِيبُه

A servant might be deprived of a provision (that was written for him) because of a sin that he commits. 

Allah said,

وَقَالَ مُوسَى إِن تَكْفُرُواْ أَنتُمْ وَمَن فِي الَارْضِ جَمِيعًا فَإِنَّ اللّهَ لَغَنِيٌّ حَمِيدٌ