How it works
Ru's account at the village bank held 20 coins, all Ru's own. With debit, the 6 coins for the book left the account at once, so it said 14.
A debit card can only spend what the account holds. With only 4 coins, the 6-coin book is a no, and nothing is owed.
With credit, the bank paid the shop 6 coins, the bank's coins, so Ru's account still said 20. A bill for 6 came at month's end, and when Ru paid it the account said 14, just like debit.
If Ru paid only 2, then 4 would still be owed, and the extra for waiting would start to grow. Neither card is a gift: each changes whose coins go first, and when.