Although purchasing agents may be able to control the timing of when they make purchases, they cannot control changes in exchange rates. Thus, it would not be appropriate to hold the purchasing agent accountable for unfavorable change in the exchange rate.
Note that exchange rate fluctuations are normally included within the direct materials price variance. Thus, multinational companies like Deere & Company (DE) should carefully investigate direct materials price variances to isolate the effects of foreign currency fluctuations.