Any slang word for "debt-collector"?( specially among gangsters & criminals!)

juice man (noun phrase, slang):

a hoodlum who collects money owed to a racketeer

'a 'juice man' (loan collector) for syndicate hoodlum bosses (1950s+ underworld)'

Source: Dictionary.com

juice collector (slang):

one who collects the money (juice) owed to a bookmaker or a loan shark

Source: Wiki

Books containing references to the above terms:

The Black Book and the Mob. Farrell, R. & Case, C.

The Rise of the Mafia. Short, M.

The Mafia Encyclopaedia, Sifakis, C. 2005

A heavy-handed collector of 'Mafia' type debts is sometimes known as a leg-breaker (per 'yourdictionary'). Another more general term would be enforcer

Shylock: slang a debt collector

The word shylock, which has been used to refer to loan sharks, is an eponym from a Jewish character in Shakespeare’s The Merchant of Venice.

A quick survey of TIME’s archives reveals 119 articles that use the word. Many of those are articles about The Merchant of Venice and Philip Roth’s book Operation Shylock — but the last time it was used casually, without reference to the character, was in a 1977 story about the mafia: “A new soldier starts at the bottom, breaking in as a senior thug’s driver, bodyguard or shylock debt collector.”

This excerpt is from When Did ‘Shylock’ Become a Slur?, which was written by Lilly Rothman for the Time Magazine Website and published on 09/17/2014. | Archival links: Archive.is/Wayback Machine

"Loan shark" -- although that would be the lender more than the collector, if the two roles are disparate.