Dear all,
I've come across this etymological proposal, but it's not the only one around.
Yule & Burnell (in Hobson-Jobson, 1903) and Dalgado (in Glossário Luso-Asiático, 1919) have a different interpretation. They mention that early Portuguese documents from Asia use a term like caxa, caxe or caixa for some low-value Asian coins, but this would not be the same caixa that means 'box' in Portuguese; it would have been a loan from a South Asian language, presumably from a Dravidian source like kasu (ultimately from Sanskrit kar,sa). And, according to that interpretation, Indian English would have received the term cash from this Asian-Portuguese word. It's difficult to tell which of the theories is correct, but Yule & Burnell and Dalgado's is perhaps relevant for this discussion group.
All the best,
Hugo Cardoso