I have no knowledge of the details concerning this matter, but like I initially said, to convert the rental or borrowing of property to a crime (theft or fraudulent conversion - whatever a local jurisdiction may legally call it) is not automatic. That conversion from a civil transaction to a criminal act requires some hoops to be jumped.
I'd imagine that selling to a third person a borrowed or rented item may be one easy hoop to jump in converting from civil to criminal in any jurisdiction. I'm glad the saxophone was located. DAVE