Transaction is not balanced

Hi All, 

Sometimes, when importing the .IMP files for purchase invoices we receive the "The transaction is not balanced" message.

It is difficult to understand the reason. As for us the Quantity*Price = Amount is well calculated(200*10.5=2100). 

Is there another reason for this message? 

Here's an example of an IMP file(Name, Address, Posta Code,... was changed):