I had a discussion at work whether the preposition following the word "discount" should be "on" or "from" or "off." The former English teacher insisted that only "on" is correct. The two lawyers agree that "from" is correct. Who is right?

Hi, Sara, and welcome aBoard. I think I would say there's a discount on that item, and a discount off the regular price, and something like you can get a discount from the other store.

Any of the following sound right to me:

The store has shoes on discount. I bought the shoes on discount for $75.

The price was discounted from $100 to $75.

The price was discounted $25 off the regular price.

...and you received a discount of $25.

I would say that there is a discount on the shoes or that the price was discounted $25. Or to make things more complicated that the price was discounted by $25. I would say that the shoes were on sale rather than on discount.
For sale meaning available for purchase, On sale meaning that the shoes were on sale at a discount.
Is there a rule or is it merely regional usage?