When sales and purchases are made on cash basis only the fluctuation in inventory days between maximum and minimum requires to be financed by overdraft.
First, that is not a rule, but is sensible management of working capital.
The minimum will be permanent working capital and would sensibly therefore be financed by long-term borrowing. The extra will be fluctuating working capital and would therefore sensibly be financed by short-term borrowing i.e. by overdraft.
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