Learn Spanish (Latin America): Pagué la cena con un billete de cien pesos. - I paid for dinner with a hundred-peso bill.

The verb is pagar (to pay). In the preterite (past) tense, the "yo" (I) form of verbs ending in -gar changes to -gué. If we just wrote pagé, the rules of Spanish pronunciation would make the "g" sound like an English "h". Adding the "u" keeps the hard "g" sound of the original verb, so it is pronounced "pah-GAY".

The translation says "paid for dinner", but I do not see a word for "for" in Spanish. Why?

In Spanish, the verb pagar directly takes the item you are buying. You just say pagué la cena (literally "I paid the dinner"). You do not need to add a translation for "for" like por.

Why does the sentence use cien pesos instead of ciento pesos?

The number exactly 100 is cien. You use cien when you have exactly one hundred of something, and it goes directly before the noun. You only use ciento for numbers over 100, like ciento uno (101) or ciento veinte (120).

Does billete always mean a paper bill or bank note?