How to round total selling price from $1,499.94 to $1,500

This is something we need to round and check it.
There are two scenarios here.

1,499.94

And

1500.09

For both cases we have to round it to 1500.

Lets workout this way.

$amount = 1499.94
$rounded_amount = round(1499.94); // 1500 here

  ov discount = rounded_amount- amount

// here this will get the discount in both negative and positive.  You should  store it on ov discount.  I guess it was not used from last updates. Use it by updating the code. If you need you can add discount gl for the discount given.

I have told the rounding trick above.  But this  unit mapping is a different concept. That we have coded here like this. Each unit will have a separate price for it and also both purchase and sales prices are available to update and use it 

This video will help you to understand it.

https://youtu.be/MM415WT-kZY

I'm trying to round total selling price from $1,499.94 to $1,500 in direct invoice under sale module but it doesn't work and I tried it in the Direct supplier invoice under purchase module and is working.

Please someone should help me on how to achieve the same result with direct invoice under sale