John Gill Commentary Numbers 18:27

"And your heave-offering shall be reckoned unto you, as though it were the grain of the threshing-floor, and as the fulness of the winepress." — Numbers 18:27 (ASV)

And [this] your heave offering
The tithe of the tithe, which though not properly heaved or lifted up, as the heave offerings, strictly so called, were, any more than the tithe itself, (Numbers 18:24); yet is so called, because separated to the use and service of the Lord, as they were:

shall be reckoned unto you, as though [it were] the corn of the
threshingfloor, and as the fulness of the winepress;
that is, it should be as acceptable to God as if they had fields and vineyards, threshing floors, and wine presses of their own, from which corn and wine were taken, as the Israelites when they received their tithes from them; and what remained they had as good a right to, and might make use of as their own, as well as they; see (Numbers 18:30).