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system
stock not available at home country nor at their local market - where should i buy it
user
theoretically, it shouldn't matter which one you use.  your return should only depend on the stock returns in sgd and the ats/sgd exchange rate (austrian schillings? is this an question from a textbook?).  whether you do the purchase "through" eur or usd shouldn't matter as the fluctuations in either currency "cancel" when you do the two part exchange sgd/xxx then xxx/ats. now, in practice, the cost of exchanging currencies might be higher in one currency or the other.  likely a tiny, tiny amount higher in eur.  there is some risk as well as you will likely have to exchange the money and then wait a day or two to buy the stock, but the risk should be broadly similar between usd and eur.
assistant