Transfer requirement will be updated

What causes this issue?

The quantity for production supply was reduced, thus prohibiting the
system from creating a transfer order. This occurred because the
requested quantity was already available in the destination storage
bin.
For transfer requirements that contain material to be picked directly
for a production order, the system reduces the quantity that is to be
transferred if there is already sufficient material in the destination
storage bin.
The transfer requirement is now considered completed.

Error message extract from SAP system. Copyright SAP SE.