You might have noticed that, after paying at a restaurant or taxi, the waiter/taxidriver will take more time than expected to get the change gathered. This normally happens because they are waiting for you to tip. If you would actually like to tip them, then, rather than getting your change first to leave them some money later, you can either say how much they can charge or how much they should give you.
An example speaks a thousand words.
Let’s say the meal/trip you just got is 45 pesos and you pay with a 100 bill (inadvisable for taxis, on the other hand). You would like to tip 4 pesos. Instead of “51 pesos está bien (para mí)”, I suggest you try with:
a. Cobrate 49 (charge 49)
b. Dame 51 nomás (give me only 51)