This is such a problem here in India, and asking someone for a receipt is often interpreted as a lack of trust. One pompous old gentleman actually collected a significant cash deposit on a land sale from my father and refused to give a receipt, saying
No question of giving receipt, my word is my honour. You can expect the land to be registered into your name within 2 days. If you doubt my integrity, you need not purchase my property.
That was 3 decades ago and his word was indeed good as gold, but over the years I have almost always needed receipts mainly for my own sense of security, so while making various sizable purchases or payments I learned to ask for one in such a way that no mistrust was implied -- most commonly by suggesting that I needed 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, and that allows both of you to "save face" in the transaction.
Note: __ "The phrase to "save face" has been around a long time. It's been part of English vernacular since the 19th century. The concept is a core social value in Asian cultures, among others. The meaning has remained stable across time. Saving Face signifies a desire -- or defines a strategy -- to avoid humiliation or embarrassment, to maintain dignity or preserve reputation."
Source: https://www.psychologytoday.com/blog/chronic-healing/201011/saving-face