I had a similar issue a few years back, I am not sure if this applies to your case though but essentially I had a Singapore Dollar credit balance in my account but found I couldn't use it, or thought I couldn't. It turned out that the Singapore Dollar balance could only be used against a purchase is Singapore Dollars, I couldn't for example use it for purchases in US dollars. It would seem any balance can only be used on purchases in the SAME currency, i.e. the seller is advertising in the same currency as you have in your account.
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