Friend: Well, let us see if that is true (that money doesn't exist). Science works by hypothesis testing, right? 

Marc: Yes, but that requires the following ordered steps: 

1.  Clearly stating one’s hypothesis. What are you trying to prove?
2. State the methods and/or experimental design and exactly how they relate to the hypothesis without assuming anything i.e. everything referred to must be objectively defined i.e. understood in a way anyone else can determine on their own. For example, cherry picking conclusions from references is pseudo science, unless the methods used to arrive at those referred conclusions are objectively evaluated and unequivocally related to one’s methods and experiment to test the hypothesis.
3. Perform the experiment in a way anyone else can replicate under the same conditions such that the hypothesis is proven or disproven identically no matter who or where the experiment is performed.   

Friend:  So interest is always a cashflow from poor to rich: confirmed by Margrit Kennedy's data and many other data. 

Marc:  No, “interest” IS ONLY an arbitrary coefficient applied to a quantity of undefined units in an arbitrary mathematical formula.   

Friend: Science?

Marc: You have presented no hypothesis, no experimental design or method and no necessary relation to the hypothesis.  Therefore, so far zero science just assertions.

Friend:  Moreover, when I have money, I go to the store and buy bread. Empirically tested. 

Marc:  Here you are making the existence of "money" contingent on the existence of a particular behaviour, without qualifying the source of the behaviour i.e. without any explanation of what existence consists of and how it can be proven,  correlation is insufficient to prove anything.

Example:

The existence of a behaviour “W”  in no way proves/tests the existence of any thing “A", unless W can be shown to be necessarily caused by the properties of A i.e. not just correlated with A.  That is, one must prove the prior (independent) existence of A in order for W to be a function of A and therefore prove the existence of A from W.

A thing must have existence first before we can attribute any behaviour to it. Things with independent existence of humans already exist, have perceivable properties that can be shown to cause associated behaviours that are functions of those properties. This is true for both physical phenomena (material objects) as well as immaterial/conceptual phenomena (logical and mathematical objects).

Therefore, things that have no relatable properties cannot be proven to "exist" i.e. frequent arbitrary correlated behaviour is insufficient to prove existence of anything.

Example: If one is conditioned to dance upon hearing a finger snap, that in no way proves the existence of dancing.  That is, while snapping of fingers proves the (prior) existence of fingers and dancing the prior existence of dancers, there are no relatable properties between the two that prove each others existence.

For the case of “money" or more precisely its unit symbol upon which money’s existence depends, without any determinable necessary physical or logical relatable properties, there is no way to SCIENTIFICALLY determine any necessary causal relationship between said symbol and any behaviour, no matter how uniform that behaviour might be.  Such that the existence of the behaviour cannot prove neither the existence of the unit symbol nor any set of conceptual (logical) properties that symbol might represent.

If said unit symbols have no independent logical or physical existence of their own and are not defined, then it necessarily follows that any associated behaviour cannot be said to be the result of the nature of said units but rather can only be the result of some other impulse.

Friend:  So, even if money is inadequately defined, it exists as a social construct that results in purchasing power in the real empirical world.

Marc:  No,  without prior existence be it by valid relatable logical intenSional definition or independent existence, "money" cannot be said to "exist", all that can be said to exist is an arbitrary symbol with no definition of what that symbol represents.  Since the manifestation of said symbol uses a varied array of physical supports, then it has no necessary physical properties either.

Things with independent physical existence of humans exist on their own merit independent of the extent of human understanding.  Things without such independent physical existence require logic and conceptual definitions to exist (Note: logic exists independently of bumans).  A symbol without any associated intensional definition can exist but represents literally NOTHING.  To say that because the symbol exists and behaviours  are made to correlate with its presence implies necessary existence of a concept, is a non-sequitur.