I mostly avoid mistrust by suggesting that I need it for my personal or organizational financial records:
Could you please give me a receipt for that Rs.12,500 payment? I need it for my files, you see, when I calculate the annual financial statement.
I need to furnish a receipt to the company to prove for their files that I actually made this purchase in the expected manner...
I need a receipt for my financial records to claim an income tax deduction.
Someone I know would even go so far as to put the blame on his wife:
My wife absolutely insists on collecting and filing receipts to know where the money is disappearing every year!
Now, what is important is not that your whatever reason should be extremely credible, but that by expressing your need for a receipt with sincere goodwill, you do indirectly manage to convey to your friend that you absolutely do not mistrust him, while also being extremely careful not to mention trust anywhere in the conversation: and that allows both of you to "save face" in the transaction.
