My brother recently received a SIS for felony burglary and is on probation for 5 years. Within the first week of his probation he took a UA and it came back dirty because he had smoked marijuana before he was put on probation. What are the possible consequences of this? Does it matter that he has not done any illegal substance since he has been on probation, and the dirty UA was from before his probation?

How does he intend to prove that the test was dirty because of activity before he was placed on probation? In my area the drug tests are all given the first day of probation so that this doesn't happen.

Often they may give him some room on the first test, but after that, he most likely has used up any favor he may get.

But at this point you talk with your PO officer and hope they will work with you. Just expect a lot closer drug checks for a while.