the situation suggests that itachi really gave the sword of totsuka to naruto.... naruto needs something that can seal edo tensei zombies..... that or naruto somehow learns the secret sealing techniques of the uzumaki's.... or nagato somehow teaches naruto some sealing techniques... nagato is uzumaki after all....
hmmm nagato is of uzumaki clan?? i dont think so, the uzumaki clan is from thw whirlpool country and nagato came somewhere far like the rain village or something, so i guess nagato may teach naruto something about defeating him but sealing techniques i doubt nagato could help naruto there...