Comma before a subordinate clause

This comma appears to be a mistake. Generally, there is no reason to insert a comma with this sentence structure. The content of the salesman's message is "I will wait if you hurry." (Of course, if we reverse the clauses, a comma is called for: "If you hurry, I will wait.")

It is possible, however, that the author inserted a comma deliberately in order to convey a brief pause that would make "if we hurried" stand out. That would slightly alter the sentence to suggest that the salesman was willing to wait, but only if we hurried. The difference would be one of emphasis, and it is rather subtle. In editing prose, I would delete the comma and see if the author objected. In editing poetry, I would talk with the author before deleting. (Sometimes the level of the author's writing skills is a determining factor.)