Is this really the story behind Black Friday?

Black Friday is the day after Thanksgiving where stores open early and reduce prices of goods to kick off the holiday shopping season. But a group condemning the participation of blacks in the sales day say it came from slavery.  Below is what they wrote 

“This was the day after Thanksgiving when slave traders would sell slaves for a discount to assist plantation owners with more helpers for the upcoming winter (for cutting and stacking fire wood, winter proofing etc.), hence the name”