All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
As stated in the beginning of the verse, Allaah, The Exalted, commanded that loans be written down and attested by witnesses in order to preserve the creditor’s right and prevent any potential disputes. However, the verse made an exception for one case when there is no harm in not recording the loan in writing so as to simplify matters for Muslims. Nevertheless, it is still recommended to have witnesses for the loan even if it is not written down.
The exception pertains to a purchase and sale transaction; when the person buys something and pays its price immediately upon taking it (into his possession), there is no need for writing this down, but it is still recommended to have the transaction attested by witnesses in order to avoid any potential disputes.
Allaah Knows best.