You can skip the next comic if you'd like to pass over the rest of this (very mildly) mature theme. I've tried to clarify the legalities as pointed out in the comments. Is Mus over or under 20 years old? How can Iusta be a witness? How many witnesses do you need to free a slave? Don't you need to perform a ceremony in front of a magistrate of some sort? And so forth. (Answers: maybe, she can't, hard to say but eventually decided to be 5, no you don't.)

From the comments:
mummpizz: Don't you need seven witnesses for a purchase as large as a slave (excluding vendor and buyer)? And aren't women forbidden to testify? Though my heart and my eyes rejoice, my brain keeps nagging.

klio: I see an eventual, much later legal requirement for five valid witnesses during an informal manumission (one done at home, rather than with a ceremony in front of a magistrate), which would indicate that the number of witnesses on such documents needed regulation.

Freeing a slave could be complex and highly ceremonial, or simple under emergency circumstances. Documents such as wills required a higher degree of formality, including being careful that highly interested parties not be the witnesses. It's not my understanding that seven witnesses are always necessary for this sort of exchange (and Felix is more concerned that someone be willing to say "I was there, he freed her before the baby was born"), but it's always possible I've forgotten something and should delve into the books and check.

I guess I should have tried harder to explain that Iusta is not meant to be signing on to the document as a witness, which she can't be like a man (otherwise Felix wouldn't have needed to find someone off the street), but as "present," just as the the women present at Iusta's birth were brought in during the lawsuit.

this comic's comments; also here.