Discount on sales value at POS in AX2012

Dear All,

Is it possible to give a discount at POS when the sales transaction meets a certain value in AX 2012. For example, if customer purchases goods of value > 300 USD, he/she is eligible for discount otherwise not.
Can we achieve this through default functionality or do we need to customize?

Secondly AX 2012 POS does not stop cashier from over tendering while declaring tender. I found Tender declaration (Max. counting difference) under Amount section of payment methods assigned to a store that it might serve the purpose. And tried it by setting a value in Max. amount difference field but it does not work. Any idea, if it works or not. If yes, how?

Any suggestion will be highly appreciated.