Keep the change
A common, slightly more indirect way. It literally means 'It's fine like that.' and is used when you don't want any money back.
A very polite and clear phrase meaning 'It's for you.' Often said after 'No, gracias.' if they try to give you change.
The informal version, using 'tú'. Use this with people you know well or who are your age, like a regular barista.
Here you are. Keep the change.
The bill is 90 pesos. I'll give you 100. That's fine, thank you.
—That's 12 dollars. —Take 15. The rest is for you.
Don't worry about the change.
'Quédese con el cambio' (formal 'usted') is the standard polite form used with service staff. 'Quédate con el cambio' is the informal 'tú' version.
'Quédese con el cambio' and 'Así está bien' are both extremely common. A shorter, more colloquial version is just 'Con eso'.
It's more common to hear 'vuelto' instead of 'cambio'. You'll often hear 'Quedate con el vuelto', using the 'vos' form.
'Quédese con el cambio' is the most common phrase. 'Así está bien' is also widely used and understood.
'Quédese con el cambio' is perfectly fine. They also use 'la vuelta' for 'change', so you might hear 'Quédate con la vuelta'.
Saying 'Guarde el cambio.' — While 'guardar' means 'to keep', in this context it sounds like you're asking them to hold the money for you. 'Quédese con el cambio' correctly implies the money is now theirs. Use 'Quédese con...' for giving a tip.
Using the wrong formality. — Using the informal 'quédate' with an older taxi driver or waiter can sound too familiar. It's always safer to use the formal 'quédese' unless you know the person well.
Confusing the noun 'cambio' with the verb 'cambiar'. — 'El cambio' is the noun for 'change' (money). 'Cambiar' is the verb 'to change'. You can't say 'Quédese con el cambiar.' The correct phrase is 'Quédese con el cambio.'