Is-that the modifications affecting the low of the balance sheet make vary the working capital? Why ?

(working capital = current assets - current liabilities) so working capital vary with affecting the low or the the high of the balance sheet. 

Absolutly, the working capital vary with affecting the low or the the high of the balance sheet because there is tow methed to calculate working capital to know through the low of the balance sheet (working capital = currents assets - current liabilities ) or  the high of the balance sheet (working capital = non currents liabilities - non current assets )

So any variation of these posts of the balance sheet affect the working capital.

No, not affecting the modification of the balance sheet very from working capital cause the owners point of view, Financial and statistical techniques by judgement should be use to predict it. Some of the items factors which need to be considered while vary from working capital requirement.