Well then explain to her that she will have to pay for that...

she is in training. that seems harsh

Lol doesn't work like that

“I’m deducting $150 from your paycheck.”

How did she managed that? Even though she is without experience she should know it’s not a free store

Fabric rings up differently than the average item. Since you can buy as many yards as you wish she probably scanned the item for a single yard instead of the amount of yards they actually had.

Something to that effect probably happened. The customer paid, just nowhere near the right amount.

Because common sense is a rare trait these days.

Unless she's being trained as a security officer, how is this something she could prevent anyway?