Abarbanel on Torah

Abarbanel on Torah, Numbers 29:32:9

There is no doubt that all of the commandments - their general principles and their details - were given to Moses at Sinai. However Moses did not immediately say them when he heard them, but rather according to the time and place that was appropriate for it - as [befits] a good leader and a praiseworthy organizer. And behold, [regarding] annulment of vows - when Moses was alive, he did not give this over to any man. Rather every man who made a vow and regretted the vow, came before him, peace be upon him, to annul it. And in his being the one expert in the generation, he would annul the vows and oaths, and clarify any doubt that existed in the matter. But when God told him (Numbers 27:12), "Ascend to Mount Abarim... and be collected to your nation," and He commanded about sacrifices that they would do after his death - since the time of his death was approaching; he saw [fit] to teach the heads of the tribes, the commandment of annulling vows which he had done himself during this lifetime to help the nation. And it is for this reason that this section is written here...