And if you are on a journey and cannot find a scribe, then a security deposit [should be] taken. And if one of you entrusts another, then let him who is entrusted discharge his trust [faithfully] and let him fear Allāh, his Lord. And do not conceal testimony, for whoever conceals it - his heart is indeed sinful, and Allāh is Knowing of what you do.
If you are travelling and do not find anyone who can write down the debt for you, then the debtor can give a property to the creditor as a security for his right until the debt is paid. If you trust one another, it is not necessary to write the debt down, call witnesses or give a promise. In such cases, the debt will be the responsibility of the debtor, which he must fulfil and be mindful of Allah regarding it by not denying it later. If he later denies it, then the person who was present at the time of the transaction must give evidence and may not hide it. If anyone should hide evidence, it indicates that his heart is sinful. Allah knows what you do, nothing is hidden from Him and He will reward you for your actions.