138 ni act friendly loan

I have bank statements of lend money also I have acknowledgement of lend money send by friend through register AD for requesting for do not deposit cheques which his gave me for security purposes. My question is its valid prove of evidence, if He written on plain blank paper with his signed. 2. Why he mentioned a security cheques, he written all content in the cheques date, name of holder, amount & his signature. 3. I am deposited cheques and get bounce with remark of payment stop by drawer.