[Avodah] All transgressions are sins?

[Daniel Eidensohn]

I am looking for a clear source that every transgression of a negative 
comandment is a sin which requires teshuva.

In particular I am studying the prohibition of ona'ah - fraud. It is 
prohibited for someone to overcharge by a sixth. If he overcharges more 
the sale is invalid while if he overcharges less than a sixth it is 
assumed that the buyer is mochel - and presumably there is no sin.

Is it necessary to do teshuva for overcharging by a sixth? The halacha 
is that if you are overcharged you have a finite amount of time to 
complain and then go to court to collect the overcharge. After that time 
has passed you can not collect in court. If you don't complain by that 
time it would seem to be assumed you are mochel. If you are mochel how 
can there be a sin?

Basically I am asking whether there is a group of prohibitions which 
when violated do not constitute sins even though they do require 
payment. Another example would be damaging another's property. What if 
you pay a person $1000 and then smash his window. Have you done a sin 
which is manifest in your soul? Or is the Torah simply saying don't do 
it and if you do you must pay - but it is not a sin.

Where does it say that violation of every negative commandent 
constitutes a sin which causes spiritual degradation that requires 
teshuva? Obviously a negative commandemnt means don't do it. But is it a 
spiritual blemish?

Daniel Eidensohn