Can I pay for cash和can I pay in cash 不一样。
正确的是： Can I pay in cash ?
我可以用现金付款吗？
分析：
pay for 为……付款
如：
1. I paid ten yuan for the book yesterday .
昨天我花十块钱买这本书。
2.I can't pay for the books now.
这些书我现在不能付款。
3 .Must I pay for my cancel?
我需要为我的取消付款吗?
拓展（表示“花费”的词还有）：
人+spend ... on sth / (in )doing sth 某人花费……做某事
某物+cost sb +金钱 某物花费某人多少钱。
It takes sb some time to do sth .花费某人多长时间做某事
如：
1). He spent ten yuan on the book.
=He spent ten yuan (in )buying the book.
= He paid ten yuan for the book.
他花了十元钱买这本书。
2). It took him half an hour to do his homework .
= He spent half an hour doing his homework.
他花了半小时做作业。