I am somewhat concerned that you admitted what you did with the sim on a document however good your intentions are. I like the way you worded it but you could have also gone down the path of
"you signed a document stating that all outstanding items have been returned" followed with "failure to return my personal property within 3 business days & all outstanding monies within 1 business day will result in a formal lodgment of theft with the Police & a claim report filed with the ombudsman"
I am no lawjiver but they are trying to fuck you over here.