Or HaChaim on Numbers 18:20/אור החיים על במדבר י״ח:כ׳

1 בארצם לא תנחל, "you will not have an in heritance in their land, etc." According to Sifri this verse forbids the priests not only a share in the land but also a share in the spoils of war. This is based on the respective meaning of נחלה and חלק both of which appear twice in our verse. The Torah tells us that though the other tribes might be willing to share the loot with the priests and they might interpret the verse as only excusing them from sharing but not forbidding same, this is not so. The legislation is not comparable to the Shemittah legislation where we are told in Shevi-it 10,9 that the rabbis were delighted if a debtor volunteered to repay a debt which the creditor could no longer insist on collecting legally. To make the point quite clear the Torah wrote לא תנחל, you must not inherit. Had the Torah written nothing else I would have thought that only inheritance of land is actually forbidden but that accepting a share of the spoils of war is permissible. This is why the Torah had to add the word וחלק, "as well as any part." The reason the Torah added the word בתוכם may be understood when we look at Proverbs 1,14: גורלך תפיל בתוכנו, i.e. "accept part of our lot together with us." The prohibition applies only when the priests receive their share as part and parcel of the division between the tribes. If anyone wishes to give them some of the spoils in the form of a gift this is permissible.

2אני חלקך ונחלתך בתוך בני ישראל, "I am your share and your inheritance amongst the children of Israel." G'd first says "I," meaning that which is unique to Him will be the share of the priests. He then added: "amongst the children of Israel," i.e. that there would not be anything the Israelites did in which G'd did not have a share; the priest would receive this share of the harvests of the land, the trees, the bread, the meat, the wine, the oil, and the remainder of the 24 categories of gifts the Israelite is to donate to the priest on G'd's behalf.