What should Amr do regarding Zayd's claim that he lent him money more than thirty years ago, while Amr does not remember that, and Zayd has no witnesses?

If Amr does not remember the amount, he is not obligated to pay it unless there is valid proof from Zayd. However, if Zayd is trustworthy and it is probable that Amr forgot due to the long period, then it is better for Amr to give him the amount or for them to reconcile.