In a letter to Malik Ashtar (RA), Imam Ali (AS) wrote:

Never betray or break your contracts, agreements and treaties, nor deceive your enemies; because no one except the ignorant and the cruel one can show such arrogance and rudeness towards God; as breaking contracts is considered an act of discourteousness/rudeness towards God. When an agreement/contract is concluded in the name of God, by Divine Mercy comfort and convenience is granted to servants of God, and as a result of this, the people will find peace and comfort, and go towards it quickly. Therefore, there is no place for corruption, betrayal, and deceit in contracts and treaties [1].

[1] Nahjul Balaghah, Sermon 53.

Source:en.shafaqna.com