Oddly enough, in financial/bank speak: a 'dead beat' is someone who pays on time or in full and thusly isn't a source of any extra $$$ than he ought to be. By being astute or wise, there isn't any extra interest or litigation to drive more money or to drive the system and create more activity thusly:dead beat.
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Let's hone in on the key:
So if someone tries to trick you into being a stooge and your failing to falling for it would fail to result in enrichment to the trickster than he ought to have, then they'd be considered to be a deadbeat (not much activity). So if say the bank knows it was paid, and you refuse to get gummed up in paying two or three or more times: if you don't go along with it then the response is exactly the same as the input: deadbeat. By using the term perhaps they are inadvertently admitting that #1 they have already been paid #2 all subsequent billing is 'extra'.
The guy who borrows $100 and pays exactly $100 back on time and before accruing ridiculous interest => deadbeat. The guy who borrows $100, gets caught up in lawsuits, incurs $10,000 in attorneys fees and a court judgement for $5,000 interest + $100 != deadbeat because his blood flowed heartily to those who sought him as a victim.