I will pay you back
Literally 'I'll pay you later.' It's used interchangeably with 'Luego te pago' and is very common.
A more formal and direct way to say 'I'll return the money to you.' Use when you want to be very clear.
Means 'I'll replace it.' Use this when you're replacing something you used or lost, which can include money.
Thanks for the coffee, I'll pay you back later.
Don't worry, my treat. — No, please. I'll pay you back the money tomorrow.
Lend me twenty pesos. I'll pay you back later, okay?
Can you lend me money for the bus? I'll pay you back tomorrow.
'Luego te pago' and 'Te pago después' are understood everywhere. Using the present tense ('pago') to talk about a near-future action is very natural and common.
'Te pago al rato' (I'll pay you in a bit) is very common and interchangeable with 'Luego te pago'.
You might hear the very informal 'Después le caigo con la plata' (I'll come by with the money later), where 'plata' is a common word for money.
'Después te pago' is standard. 'Plata' is the most common word for money, so you will often hear 'Te devuelvo la plata'.
While 'Luego te pago' is used, 'Ya te pagaré' (using the future tense) is also very common. They also frequently use 'devolver', as in 'Te lo devuelvo mañana' (I'll return it to you tomorrow).
Pagar para atrás — This is a direct translation of 'pay back' and is incorrect. In Spanish, the concept of 'back' is already included in verbs like 'pagar' (to pay) or 'devolver' (to return). Correct: 'Te pago' or 'Te devuelvo el dinero'.
Overusing the formal future tense — While 'Te pagaré' (I will pay you) is grammatically correct, it can sound a bit stiff in casual conversation. It's much more natural to use the present tense ('Luego te pago') or the informal future ('Te voy a pagar') for near-future plans.
Using 'espalda' for 'back' — Some learners might try to translate 'back' literally as 'espalda' (the body part), leading to nonsensical phrases. Remember, the 'back' in 'pay back' is an adverbial concept, not a noun.