Going to Market

A shopkeeper has lent 50 dinars to a sheik, and another 50 to a merchant. The sheik paid the debt in four installments of 20, 15, 10, and 5 dinars:

Installment	Paid		Still Owed
1	20		30
2	15		15
3	10		5
4	5		0
Total	50	Total	50

 
The merchant paid the debt in four installments of 20, 18, 3, and 9 dinars:

Installment	Paid		Still Owed
1	20		30
2	18		12
3	3		9
4	9		0
Total	50	Total	51

 
How to explain the difference of 1 dinar?

Mathematical excerpts from The Man Who Counted by Malba Tahan