The supply line is in exception because the quantity was reduced in the execution document

Hello,

I'm trying to create make order through sales order using Back-to-Back. I'm facing Following error,

The supply line is in exception because the quantity was reduced in the execution document.

Note: The item has dual unit of measure

2000 EACH = 2 UPS

Can someone confirm, what this error means.

Thanks in advance.

Regards