Wǒ fù xiānjìn.

我付现金。

I'll pay by cash.


Notes:

我(wǒ): I

付(fù): to pay

现金(xiānjìn): cash

Example:

A：你付现金吗？

     Nǐ fù xiānjìn mɑ？

     Will you pay by cash?

B：我付现金。

     Wǒ fù xiānjìn.

     I'll pay by cash.