How is a document written to prove that a wife borrowed an interest-free loan from her husband without specifying a repayment period?

Writing down debt is commendable as a means of securing and preserving rights. It is not obligatory, so one who omits it does not incur sin, whether the debtor is a husband or someone else. Some scholars have held that writing down debt is obligatory, based on Allah's saying: "then write it down." However, the correct view, held by the majority of jurists, is that it is not obligatory. There is no specific prescribed legal format for writing down debt, so the husband can write whatever confirms the establishment of the right in his obligation to his wife.