What might be one important consequence of a law that required any slave caught running toward Canada and freedom be returned to its master?

Hi, you've asked an incomplete question. However, I inferred you want to know the effect of the Fugitive Slave Acts.

some free blacks were illegally sold into slavery

Explanation:

These series of modified laws often created the opportunity for some to deliberately capture free black men, with false accusations that they were slaves running toward  Canada. Which in reality wasn't the case, but they were mere victims of "kidnapping".

For example, the consequence of this law proved through in the case of Solomon Northup, who was captured under the disguised that he was a runaway slave until he regained his freedom after 12 years of slavery.

where is the section with the question?