John Gill Commentary Numbers 5:7

"then he shall confess his sin which he hath done: and he shall make restitution for his guilt in full, and add unto it the fifth part thereof, and give it unto him in respect of whom he hath been guilty." — Numbers 5:7 (ASV)

Then they shall confess their sin which they have done
The form of which confession, according to Fagius, was, O Lord, I am guilty of death, I have deserved to be stoned for this sin, or to be strangled for this trespass, or to be burnt for this crime

and he shall recompense his trespass with the principal of it, and
add unto it the fifth [part] of it ;
paying the whole of whatsoever he had in any manner defrauded his neighbour of, to which he was to add a fifth part of that; that is, as Aben Ezra interprets it, if he confesses of himself, but if there are witnesses of it he must add two fifths, and some say a fifth of a fifth:

and give [it] unto [him] against whom he has trespassed ;
as a satisfaction for the injury done him.