You made me laugh 
It is a national tradition, although I cannot endorse it 100%, it is very likely that the lack of money in the last days of the month has contributed to this tradition that is ours.
Just a few days ago I heard an Argentine journalist who is in Spain and also traveled to Italy and asked if the history of gnocchi there also had that curious tradition of putting money under the plate but it seems that is not the case.
If the Italians make exquisite "gnocchi" with a variety of sauces that are the envy of the gastronomic world, but putting money under the plate and eating them every 29th of the month, no, that is exclusive to this particular country, so crazy. as dear.
RE: What's a Ñoqui? 🍝 Giving it a Try.