John Gill Commentary Numbers 18:19

"All the heave-offerings of the holy things, which the children of Israel offer unto Jehovah, have I given thee, and thy sons and thy daughters with thee, as a portion for ever: it is a covenant of salt for ever before Jehovah unto thee and to thy seed with thee." — Numbers 18:19 (ASV)

All the heave offerings of the holy things
All before mentioned, and whatsoever comes under that name:

which the children of Israel offer unto the Lord ;
either by his commandment, or of their own freewill;

have I given you, and your sons, and your daughters with you, by a
statute for ever :
so providing for their maintenance by an irrepealable law as long as their priesthood lasted, even until the Messiah should come:

it [is] a covenant of salt for ever before the Lord unto you, and your
seed with you :
an incorruptible, inviolable, durable covenant, which should last for ever, even until the Gospel dispensation or world to come should take place; and it would remain ever before the Lord in his sight, who would take care it should never be made void, but stand fast with Aaron and his posterity as long as his priesthood endured.