My friend is asking again—He was working in a
restaurant, serving everything except wine (khamr)but
the meat there is not halal and he knows that; he was
carrying the empty glasses of wine and washing them,
he knows that that is not allowed. His Q is: he felt guilty
so he left the restaurant. He had money out of that job
and he wants to start a new halal business.

CAN HE USE THAT MONEY TO START A NEW
JOB? IF HIS MONEY IS NOT HALAL, DOES HE
NEED TO GET RID OF THAT MONEY OR CAN HE
WORK WITH IT AND FROM THE PROFIT HE
CAN TAKE THE SAME AMOUNT OF MONEY?
WAJAZAKUM ALLAH KHAYRAYAMAN

The job which this brother was performing is the kind in which halal
and haram have been mixed together. At the same time that he was
serving food that contained haram ingredients such as pork or lahm
ul-maita (meat not properly slaughtered), he was serving things that
are halal as well washing dishes and glasses. If he had known at the
time he was working about the prohibition of the haram things, then
his income in this situation comprises mixed money, both halal and
haram. So if he wants to repent to Allah, then he needs to get rid of
the portion of his income which his haram, by estimating and
approximating to his best judgment how much the haram portion of
the income was which was earned via haram service. He can spend it
in any of the various charitable means, and he cannot benefit from it
whether by investment or otherwise. As for the remaining portion
which is the halal amount, he is allowed to use it to his benefit.