Massacre of Verden year 782

Supposedly 4,500 Saxons were put to death by Charlemagne.

This number is accepted by most historians but

1) What was the population of the area? And how many soldiers did Charlemagne have in Verden?

2) Charlemagne had a slave trade going with the Caliph of Bagdad (Harun al-Rashid). Trafficking them to Bagdad would have made more sense.

I feel like your post is in the middle of a much larger conversation

Please explain.