yì yán wéi dìng

一言为定 (一言為定)

settle something with one word

it’s a deal; that settles it; that’s settled then

Example:

一点儿不错, 就这样办, 一言为定。
yīdiǎnr bùcuò, jiù zhèyàng bàn, yīyánwéidìng.
Absolutely right. That’s the way to do it. So it’s settled.