Prevent salesPrice updating when salesQty is chnage

HI.

For some saleslines. I would like to  prevent salesPrice updating when salesQty is chnaged

How can I do that?

I have check the Accounts receivable parameters, On the "Prices" page tab, under the "Trade agreement evaluation" heading, add "Manual entry" , but it didn't show dialog after I update SalesQty.

So can I achieve this with x++? 

thanks