This scene is very short and happens just after Bassanio has sent Gratianio with the ring for Portia. He catches up with Portia and tells her that Bassanio has given her the ring and wants her to join them for dinner.
Nerrissa, seeing that Bassanio has parted with the ring decides to test Gratiano to see if he will give up the ring, which she gave him. Again Dramatic Irony is at play here, as Gratiano is unaware that it is Nerissa, dressed as the lawyer’s clerk.
Portia tells Nerissa that they will have a lot of fun when the men swear that they gave the rings to men. Nerissa leaves, going to Shylock’s house with the deed to be signed.
Enda's English Notes 