Learn English – single word for “money-mindedness”

The word I was looking for was parrhesia.

Parrhesia is defined as boldness of speech. But it's a great word with another special meaning:

Quoting from 1000 Most Challenging Words, by Norman W. Schur (Galahad Books, 1987):

In oratorical rhetoric, i.e., the art of influencing an audience, parrhesia, in the words of William Safire (On Language, in The New York Times Magazine of October 21, 1984), "has a specialized meaning: 'warning of potential offense, and asking pardon in advance.'" The expression with all due respect (in Britain, they shorten it to with respect) is an example of parrhesia: What it really means is, "I haven't the slightest respect for you and certainly not for what you just said, and I'm going to show you up before this prestigious audience for the blithering idiot you are...!"

The word comes from the Greek prefix para- (beside, beyond - as in, e.g., parapsychology) plus rhesis (speech).

Other examples include "Forgive me, but...", "Pardon me, but...".

At least this gives the recipient of such a preface a short time to best prepare for the oratorical firestorm coming their way!

As already suggested by the learned members 'remnant' and 'Hellion' in the earlier answer and comments, both 'savant' and 'idiot savant' could be ideal expressions for your purposes.

It seems that 'idiot savant' (literally meaning 'very knowledgeable idiot' from the French) would be a paradoxical intensification of 'savant', because the sense of being less competent in most areas other than that of special ability is already embedded in the meaning of 'savant' as quoted in the earlier answer by remnant.

However, there seems to be rather a fine difference, in that a 'savant' would be very learned at something but not necessarily idiotic in all other matters; whereas an 'idiot savant' is indeed an extreme form (of savant) in being exceptionally weak at everything except the special talent! Some people consider that 'savant' carries positive connotations as it simply means 'highly gifted/ learned in some specific area', but 'idiot savant' carries negative connotations because of the assumption of extreme ignorance/ incompetence in most areas except the gifted field.

On the other hand, 'specialist', while certainly implying special expertise in some area, does not create the sense of bring incompetent in other fields.

Definition of savant (by Merriam-Webster online dictionary)

1: a person of learning; especially : one with detailed knowledge in some specialized field (as of science or literature)

2: a person affected with a mental disability (such as autism or mental retardation) who exhibits exceptional skill or brilliance in some limited field (such as mathematics or music); especially : autistic savant

https://www.merriam-webster.com/dictionary/savant

Merriam-Webster defines 'idiot savant' as follows (originally quoted by the senior member Hellion in comments)

Definition of idiot savant

1: savant (see above definition)

2: a person who is highly knowledgeable about one subject but knows little about anything else (which corresponds almost exactly to your own description of the word you seek!)

https://www.merriam-webster.com/dictionary/idiot%20savant

Random examples:

"Matt is just your everyman most times, and quite competent at most things; but ask about the etymology of obscure English verbs on ELU and he becomes a savant!"

Joe is unbelievably good at writing brilliant software for any situation, but disastrous at the simplest task otherwise: he is (almost) an idiot savant.