There are two ways of answering this question:
- Look at early meanings of 賺 as the individual character
- Look at early meanings of 賺錢 as the whole phrase
The character 賺 (Middle Chinese: /ɖˠɛmH/ , Old Chinese: /*r'eːms/) may have originally represented a word which eventually diverged into several other characters (詀, 譧), all with the related meaning crooked/improper/dishonest words. In this sense, 賺 may have been originally extended from a meaning to gain a profit by swindling somebody else. We can see the original meaning resurfacing somewhat in several quotes in classical texts:
Here, 賺 means to mislead or more generally have a negative impact on.
The other common earlier meaning of 賺 was to gain/acquire/win (a profit, resources, etc). This is the most direct meaning to 賺錢, which was originally a vernacular word.
Based on the above, you can either take
賺 on etymology, in which case you can interpret it to mean to gain a profit off of someone else by cheating them. However, I think this interpretation is inappropriate, because by the time 賺錢 appears as a complete, set phrase,
賺 just overwhelmingly meant to gain/acquire/win, so the literal meaning of 賺錢 should really be taken as to gain/acquire/win money/wealth.