How can the issue of the $100 deficit between Abu Malik and Abu Nour be resolved, especially after the emergence of a video proving that Abu Ahmad received $2,100 instead of $2,000, and the deficit was confirmed from Abu Nour's fund?

The dispute here is between the seller and Abu Ahmad, who has the right to sue Abu Malik, and Abu Malik has the right to sue the one from whom he took the amount. The video is a valid piece of evidence for proof, and the judge may rule based on it in cases other than hudud punishments, if its authenticity and clarity are established. If Abu Ahmad is certain of the video's clarity and that it shows him receiving the extra hundred, he is religiously obligated to return it to the seller.

If the matter reaches the judiciary, the judge may rule based on the video as clear circumstantial evidence. If the judge does not rule based on it, or if the video is not clear, and the seller has no other proof, Abu Ahmad will be asked to swear an oath that he did not receive the extra hundred. If he refuses to swear, he will be judged to have defaulted (nukul) and ordered to pay it, for "the burden of proof is on the claimant, and the oath is on the defendant."