What is the Sharia ruling for resolving a dispute between two friends regarding the payment of 2000 Riyals, where one claims to have paid it and the other denies it, with no evidence for either of them?

The decisive word in lawsuits and disputes for the Sharia courts. If you acknowledge the principal of the debt, and you both differ on whether payment has been made or not, then the principal is that the debt remains, and you are obligated to pay it, unless there is evidence of payment, because the default is non-existence. The proof for this is the saying of the Prophet, peace and blessings be upon him: "Were people to be given according to their claims, some would claim the blood and wealth of men, but the oath is upon the defendant." And "The burden of proof is on the claimant, and the oath is on the one who denies."