On the authority of Abu Hurairah (may Allah be pleased with him) who said that the Messenger of Allah (peace be upon him) said: "Render back the trust to those to whom they are due and do not betray the person who betrays you" [Narrated by Abu Dawûd, Hadith no. 3535 and At-Tirmidhi, Hadith no. 1246]
"Render back the trust" is a commandment that must be implemented. Allah [may He be Glorified] said: "Verily! Allâh commands that you should render back the trusts to those to whom they are due." (4:58)
"Trust" means everything that must be fulfilled.
"do not betray the person who betrays you" means that a Muslim should not mistreat people even though they might have mistreated him; and thus he should not betray those who might have betrayed him.
Furthermore, this is a comprehensive commandment for rendering back the trust, which can be something borrowed, a deposit or anything else.
The Hadith also directs people not to return mistreatment with similar conduct. This issue is disputable among the scholars, but forgiving mistreatment takes priority, as Allah [may He be Glorified] said: "The recompense for an evil is an evil like thereof; but whoever forgives and makes reconciliation, his reward is with Allâh. Verily, He likes not the Zâlimûn (oppressors, polytheists, and wrong-doers ." (42:40)
[See Subul As-Salam, Vol. III, Hadith no. 89]