Commentary for Numbers 18:27

ונחשב לכם תרומתכם כדגן מן הגרן AND THIS YOUR HEAVE OFFERING SHALL BE RECKONED UNTO YOU AS THOUGH IT WERE THE CORN OF THE THRESHING FLOOR — i.e. the heave-offering of your tithe is forbidden to laymen and to unclean priests, and such of these as eat it of set purpose incur the death penalty, or, if they have eaten it inadvertently, pay in addition to the capital sum a fifth, just as in the case of “T’rumah Gedolah” (the T’rumah set apart by the ordinary Israelite) which is termed the “firstling of the corn” from the threshing floor.

ונחשב לכם תרומתכם, to ensure that blessing will be present in your homes even though we are speaking about something which has been measured, something not ordinarily subject to blessings. Our sages had gone as far as saying that when someone prays for G’d’s blessing on things he had already counted, measured, or weighed, this is a futile prayer, i.e. he uttered the Lord’s name in vain. (Taanit 5) This תרומת מעשר, tithe of the tithe is exempt from such restrictions.

כדגן מן הגורן; forbidden , but not on pain of death, under the heading “no non-priest must eat t’rumah in Leviticus 22,15-16.