Price change not allowed, due to negative stock

What causes this issue?

You want to change the price of a material.
You cannot change the price in the current period, as negative stock
exists.
You cannot change the price in a period which is not the current period
because:
either the stock in the period selected is negative
or the stock in the current period is negative
or the stock in the selected period and in the current period is
negative

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.