It's not so much that you have to prove a negative--that there was no guaranty--but that he has to prove by a "preponderance of the evidence," or that it is more likely than not, that you *did* provide a guaranty; as the person suing, the burden or obligation is on him to prove his case, and if he can't prove it, he loses.He can try to prove it via his oral testimony, but he might not even be able to get that in: for him to testify what you said to him is technically hearsay and may be inadmissable. Even if he can get what he claims you said in under an evidentiary exception to hearsy, if there is otherwise a written agreement between you, almost no court would hold that an oral guaranty ("oral" is the better term than "verbal") could be added a written agreement: written agreements almost always have to be modified in writing. And even if there is no written agreement and he can testify as to what he claims you said or offered, you can provide your contrary testimony, and since he must affirmatively prove your case, unless he were markedly more credible than you, it's unlikely he could do so. All in all, what you describe is a weak case for him at best.