Apply Automatic Hold if Unit selling price different with list price

Dear Guru's,

There is requirement from my retail client if, selling price is different with list price

system will automatic HOLD the sales orders.

Example: User fill in unit selling price for item A in sales order line= 2$, but actual list price in price list = 3$

in this scenario, override price list is enabled.

Any suggestion how to achieve this requirement?

Thank you a lot

Made S