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.