Why should people abstain themselves from dishonesty (such as bribe) and spend money for the benefit of others?

 

Answer:

Verse 188 of Chapter 2:

Do not usurp one another's property by unjust means nor offer it to the judges so that you may devour knowingly and unjustly a portion of the goods of others.
 
Verse 195 of Chapter 2:
Spend your wealth in the Way of Allah and do not cast yourselves into ruin with your own hands. Do all things gracefully, for Allah loves those who do all things with excellence.

Verses 29-32 of Chapter 4:

O Believers, do not devour one another's property by unlawful ways; (instead of this) do business with mutual consents And do not kill yourselves; believe it that Allah is Compassionate to you. We shall throw into the Fire the one who commits such acts of aggression and injustice; and this is very easy for Allah. If you abstain from the heinous sins from which you are being forbidden, We will remit your trivial offenses and admit you to a place of honor. And do not covet what Allah has given some of you more than others: the men shall have their due share according to what they have earned and the women shall have their share according to what they have earned. So pray to Allah for His bounty; most surely Allah has perfect knowledge of everything.

Verses 58 of Chapter 4:

(O Muslims), Allah enjoins you to give the trusts into the care of those persons who are worthy of trust and to judge with justice, when you judge between the people. Excellent is the counsel that Allah gives you, for Allah hears everything and sees everything.

Verses 8 of Chapter 5:

O Believers, be steadfast in righteousness and just in giving witnesses for the sake of Allah; the enmity of any people should not so provoke you as to turn you away from justice. Do justice for it is akin to piety. Fear Allah (in your affairs): indeed He is fully aware of what you do.