De articulis liberalis causae liber singularis
The Same, Actions Relating to Freedom. Anyone who knowingly purchases a man who is free, even if the latter permits himself to be sold, cannot, nevertheless, oppose him, if he demands his freedom. Where, however, he sells the man to another person who was ignorant of the facts, the supposed slave will not be permitted to demand his liberty.
Paulus, Articles Referring to Actions for Freedom. If there is any doubt as to the condition of a person who demands his freedom, he should first be heard, if he wishes to prove that he himself is in possession of freedom. 1The judge who has jurisdiction of cases where freedom is involved should also take cognizance of property which has been stolen, or serious damage committed by the claimant. For it can happen that, being confident that he will obtain his freedom, he may have ventured to steal, or spoil, or waste property belonging to those whom he was serving as a slave.