Last act... how does Portia work within the man made law to give effect to the higher law?

Portia finally clears up the confusion by informing Bassanio that she and Nerissa were the doctor and the clerk. She further has good news for Antonio, namely a letter that indicates that three of his ships arrived in port safely. Nerissa then hands Lorenzo the deed from Shylock in which he inherits everything after Shylock dies. The play ends with Graziano promising to forever keep Nerissa's ring safe.