According to Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), what is meant by the words "Make a contract with them if you know any good in them" [Surah al-Nur: 33] is trustworthiness and fulfilment (of obligations).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21604