Others here can advise on specific classic Fiat 500 details, as the last one I owned was in 1978.
However from a mechanical point of view, first what oil is leaking, if engine oil, then down to their rebuild. Especially if leaking from sealing arrangement behind flywheel or sump gasket sealing in that area.
If gear oil harder for you to prove that no gear oil leaks prior to engine work.
Do you have photos of clean dry gearbox just prior to going to garage.
Is the gear oil leaking from engine end, in which case there may be a slight argument of damage when pushing engine back onto gearbox during refitting maybe damaging area oil seal on clutch release bearing slides on tube , but very hard to prove unless obviously broken.
If oil leaking from drive shaft area, was the gearbox left in place and just engine removed for the work?
If driveshafts were disturbed and proof it was dry before engine job maybe a question.
So back to my original point, check if oil is from engine (should be be clean and the colour of new chip oil if engine just rebuilt) if gear oil it is usually slightly darker, a thicker consistency and stronger smelling) so to sum up, if it is engine oil and you have just paid for a complete rebuild then down to garage to do work at their cost. If gear oil most likely at your cost as hard to prove it wasn't there before they worked on vehicle.
Note this is just my opinion and not "on site" to inspect, so a guess.