"Due" means that an amount of money is owed. When you receive your electricity bill, for example, it might show "Amount due: $50". This is the amount you have to pay.
The vicar puts all of his bills in the drawer marked "Due unto others" to show that it contains details of money that he must pay other people.
"Due unto others" sounds very similar to "Do unto others" - the pronunciation of the first word is different, but when combined with the rest of the phrase, the pun is clear.
Please note: You have posted many of these questions, and each time you type "humur" in the title. The correct spelling is "humour" in BrE and "humor" in AmE.
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