(Standard disclaimer: I am not your lawyer; I am not here to help you.)
Under American common law, the distinction here would relate to the harm to B: either a damages issue or a "special harm" issue.
The Restatement elements of defamation are falsity, publication, fault, and inherent actionability or special harm. See Rest. 2d Torts § 558. The last element captures the traditional doctrine that slander (not libel) is only actionable if it falls into one of four or five specific categories ("slander per se"), or if it actually causes economic injury. Your example doesn't seem to fit into any of the special categories. But see Rest. 2d Torts § 573 (imputations affecting business or office).
If the statement to C is oral rather than written, and C doesn't believe it or otherwise nothing comes of the statement, B may not be able to prove special harm and therefore fail to recover anything. If D, on the contrary, avoided doing business with B, B may be able to show special harm supporting a claim.
Similarly, C's disbelief or D's belief may be relevant to determining the actual damages B suffered and is therefore entitled to recover from A.