paraphrasing

It means that if his helper goes to Washington, Mr. Khoshtip won't need to.

Thanks but I don't understand it still! My problem is about "to go" (end of the sentence). What does this mean? Or what is the complete version of it?

Complete version: to go to Washington.

Does that "to" needed in the sentence: "to go to Washington." Couldn't we leave it out?

It would be fine to end the sentence at 'volunteers'.

OK but what about what I asked please, the first "to" after "unless"? Is it needed or we can say the sentence this way:
Mr. Khoshtip has to go to Washington on a business trip unless his associate volunteers to go.

khoshtipMr. Khoshtip has to go to Washington on a business trip unless his associate volunteers to go.

You need the 'to' between 'volunteers' and 'go'.

khoshtipunless his associate volunteers to go.

More clearly:

... unless one of his associates volunteers to go.

Note that "volunteers" is a verb, not a noun. At first I read it as a noun, and that led me to think the sentence was ungrammatical.

With volunteer as a noun:

... unless one of his associates offe

khoshtipunless his associate volunteers to go.

With volunteers as a noun this is OK without 'to'. However, the expression associate volunteer is not likely to refer to any real entity. There is no such thing as a principal volunteer, an assistant volunteer, or an associate volunteer. That's why Philip and I are steering you away fr

CalifJimNote that "volunteers" is a verb, not a noun.

It was the key. Thanks.
I would think volunteers is a noun so I'd think to is not needed and that was why I asked that question from Philip.
Now I understand why to is required.
But, I think the sentence with volunteers as a noun/Adj hadn't problem!