John Gill Commentary 1 Samuel 2:16

"And if the man said unto him, They will surely burn the fat first, and then take as much as thy soul desireth; then he would say, Nay, but thou shalt give it me now: and if not, I will take it by force." — 1 Samuel 2:16 (ASV)

And if any man said to him, let them not fail to burn the
fat presently
Or stay till they have offered the fat, as the Targum; let that be done in the first place, which may be quickly done, in a very little time, and let as much haste be made as can be to do it:

and then take as much as your soul desires ;

by which it appears that the men that brought the sacrifice had more religion at heart, and were more concerned for the honour and glory of God than the priest. Being willing to suffer in their property, but could not bear that the Lord should be dishonoured, and so rudely treated. They were willing the priests should take what they pleased of theirs, though they had no right to any; only they desired the Lord might be served first, which was but reasonable:

then he would answer him, nay, but you shall give it me now, and if
not, I will take it by force ;

signifying, he would not stay till the fat was burnt, and the Lord had his portion, but he would have it directly; and if he would not give it him freely, he would take it whether he would or not. To such a height of insolence and impiety were the priests arrived, as to put it in the power of their servants to make such wicked demands, and treat God, and those that brought their sacrifices to him, in such a contemptuous manner.