Is it one yuan or two yuan?

一元还是二元？

I lost one yuan.

我丢了一元钱。

This account is wrong; we're one yuan short.

账算错了，少一块钱。

Teacher:I picked up one yuan.

老师：我捡到一元钱。

I give one yuan for ice cream.

我花一元钱买冰激凌。