A friend of mine a cashier was short of cash so he so he callled me to collect some money from his friend and send it to him,I never know that he is having a shortage and I am being accused of being covering for him I don't know what to write to defend myself?

Sounds like the Friend may be setting you up, Got you involved to add doubt.
If I wanted to make sure the Money Was not short and I was responsible for it, I would not let a 3rd party handle it. Are you also an Employee?