Green’s Dictionary of Slang

marker n.2

[it has been ‘marked down’; see Asbury, Sucker’s Progress (1938) 17: ‘An extraordinary number of the terms, technical and otherwise, which were employed by Faro players in the palmy days of the game have passed into the language [...] Marker – An article, sometimes a small piece of ivory provided by the bank, used by a player whose credit was good. He announced the value of the marker as he made his bet, and was supposed to settle after each deal’]

1. [late 19C+] (orig. US) an IOU for a gambling debt; also used fig. for any form of debt.

2. [2000s] (US Und.) money.

In phrases

call in one’s marker (v.) (also call in a marker, ...someone’s marker)

[1970s+] to demand repayment of a favour.

put out a marker (v.)

[2000s] (US Und.) to send out a call; to offer a reward.