1. Navigate to the Inventory > Products > Products

2. Select the product

3. Click on Add a Line to add the vendor for the product in the Purchase tab

4. Add the special price for that vendor and the currency

5. Now that the special price has been updated, when purchasing the specified product from the specified supplier, the specified currency will be applied. 

You can also watch this video for reference: https://waoconnect.screencasthost.com/watch/c0enInVyLrO