Sa'id bin al-Musayyib (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) said: "The pledge is not to be locked away." Thereby the owner of the pledge is prevented from buying from the one with whom he pledged it, until he buys from someone else. This is how I found it in my book, and the correct wording, as I think, is: Locking the pledge means that the owner of the pledge is prevented from buying from the one with whom he pledged it until he buys from someone else; so he said: "The pledge is not to be locked away", meaning the owner of the pledge is not to be prevented from trading with the one holding the pledge. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرهن / 11237
Mu'awiyah bin Abdullah bin Ja'far (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) said: "The pledge is not to be locked away." A man pledged a house in Madinah for a fixed term, and when the time of payment came, the one who had taken the pledge said: Now this house is mine. The Messenger of Allah (peace and blessings be upon him) said: "The pledge is not to be withheld." This is mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرهن / 11238
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