Ma'mar (may Allah have mercy on him) said: I asked Imam al-Zuhri (may Allah have mercy on him) about the meaning of "La yughlaq al-rahn" (the pledge is not locked). He said: "It means that a man says: If I cannot pay by such-and-such a day, this pledged item will be yours."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرهن / 11239