The RAE's ever-helpful Diccionario de americanismos gives us:

"I. 1. f. Mx, Gu, Ho, ES, Ni. juv. Dinero suelto, cambio.
2. Mx, Gu. Dinero, moneda corriente. pop + cult → espon.
II. 1. f. ES. Cárcel."

I think we can discount definition II. Definition I.1, juvenile slang for loose change, seems to be used in Mexico and much of central America. I.2, slang for money, is said to be used in Mexico and Guatemala, but as with "morra" in a previous question, it seems to be used in El Salvador too, or at least (as Laura suggests) it forms part of the Maras' vocabulary.

This is plain from the following which reports verbatim the threatening messages a bus driver/operator received from the Maras extorting money. It's one of several slang terms they use for money. They're clearly into lexical variety:

"La onda es que queremos pisto, $5,000 lucas [...]
si no nos das el “money” te vamos a matar [...]
sabemos donde estudian tus hijos y si no queres que se queden sin tata danos la plata [...]
necesitamos el pisto y si queres vivir tenes que “ayudarnos”. [...]
Te vamos a llamar mañana para que nos digas cuándo nos vas a dar la “feria”."
http://www.lapagina.com.sv/nacionales/81020/2013/04/29/He-pa...$5-mil-mensuales-a-pandilleros-por-la-seguridad-de-mi-familia

Note, by the way, that "feria" is given in inverted commas here, as though it's not standard in El Salvador. But the meaning is surely unmistakable. Not change, but just money.


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Note added at 1 hr (2014-08-14 22:31:07 GMT)
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Here's an alternative link to the dialogue in case the first one doesn't work:
http://tinyurl.com/p2aq2sd


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Note added at 1 hr (2014-08-14 22:37:11 GMT)
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One point to bear in mind is that the idiom of the Maras draws on the vocabulary of gang cultures in other countries. They talk about "money" and "homeboys". I think it's perfectly plausible that they would used Mexicanisms, given the huge criminal power and prestige of the Mexican narcos.

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Note added at 1 hr (2014-08-14 22:49:36 GMT)
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A couple more examples of feria = money in El Salvador. This is from a blog discussion called "Despendejizando al Pueblo Salvadoreño":

"Hereje salvadoreño. dijo...
Según le oí decir a unos de los asesores -en materia económica- de Funes, que la única vía para poder hacer llegar la feria a las arcas del Estado es, subir los impuestos. [...]
"-okey, tengo que dejarte porque vua atender a unos vatos de china que me han venido a cobrar una feria que les debemos
-esperate varac, a proposito de feria, cres que me podes prestar unas chirilicas, es que fijate que estamos sin plata [...]".
http://cuxcatla.blogspot.com.es/2011/02/despendejizando-al-p...

And another:

"pooota loko lo mismo iva a poner yo bro....la onda chero javialianza..de el salvador y la usa....haya para la mara de ...
el salvador ..4 0 5 dolares es mucha feria..loko..."
http://lacasablanca.activeboard.com/mobile.spark?p=topic&top...