Hi! Are you talking about giving someone a tip? This is a must in American culture. If so you could say a few things: "Could I have 40 dollars back? Please keep the 10 as a tip." Or a better way I think is to have them give you the change for the purchase. So they give you back 50$. Then you leave a 10 dollar tip on the table.

You want to leave a $10 tip? Then ask the waiter to bring you $40 in change, or ask if you can get back $40 from this. Usually we just wait for the change and leave the $10 on the table. Hope that helps.

While perhaps grammatically correct, that would sound rather odd. I would normally say something like “Thanks for the excellent service, just go right ahead and make it sixty bucks.” Some people would say “Thanks for the excellent service, just bring me back forty bucks in change,” Note that, in the US, a 20% tip is appropriate for good service -- I say "excellent" just to be extra nice -- it doesn't cost a thing ;)