In The Force Awakens, the big fat junkboss unkar Plutt was seen trading these metal-like "portions" to the scavengers of Jakku. What was the difference between these and some simple hunks of metal? And why is he acting so cruel to rey? On the first day something was worth a certain amount of money and then the next it was only a fraction of the price. Did she do something to hurt him before or is he just an overall douche?
They are "portions" of food.
From the novelization:
Somehow she managed to restrain her reaction to a single muscular twitch. Sixty portions would feed her for… for… for a very long time.
“Oh, I’m not going to keep him for myself.” Plutt spoke absently as he continued to stack full nutrition portions beside his seat.
Unkar Plutt appears to just be a bit of a lech.
“That’s my girl,” Plutt commended her. Not replying, she turned and left, moving as quickly as she could without alerting him to the fact that his presence disgusted her. She could feel his eyes all over her until she exited the big tent.
“That’s my girl.” His tone oozed something more than false possessiveness. There was an eagerness in his voice that was something new even for Unkar Plutt. An eagerness that all but translated into triumph.
Additionally, he seems to just enjoy lording control.
Unkar Plutt, on the other hand, was delighted to extend their encounters for as long as she could stand it. He always took his time when examining her pieces, letting his gaze rove slowly over everything she put before him, making her wait. Only when the bounds of common courtesy had been markedly surpassed did he deign to acknowledge her presence.
Plutt watched her go. He was starting to calm down, his mind working systematically. The confrontation had almost escalated beyond repair. Such loss of control was not like him. In the course of negotiations he would often shout, yell, occasionally pound the service shelf in front of him. But all the time, he was calculating. It was all about the business, all about the profit. Never personal. Not even now, when it involved the lovely but disrespectful Rey. That was something of a pity, he mused as he picked up a communicator.