Whale,
Last year I was able to pay OWE related reissue charges in USD to BA in EZE.
I think it is a well established that BA in CAI charges your card directly in EUR or USD so asking if they would accept cash is reasonable to me. In addition, the same currency collapse that has made these tickets so cheap, has probably lead to wide spread acceptance of the USD and EUR through out the country.
If you try the cash route let us know how it goes.
rich