What is the best control over foreign currency price paid by company to traders, in circumstance of shortage in foreign currency supplied by banks ?

By following a pricing policy using a Stable and reliable Foreign Currency in exchange of Export proceeds.  When inflationary pressures are mounting this will also have certain advantage at the time retirement of your bills by the foreign correspondent.  The control and caution can be extended by opening one "Exchange Earners's Foreign Currency Account" with the bank and keep the proceeds in the account, till it is having a better Exchange rate both for en -cashment purpose and for meeting payment obligations.

its impossible to control the exchange rate but you could limit your representative privileges in case of shortage of foreign currency which is also highly unpredictable by setting your Owen preserve of foreign currency or you could adopt a flexible policy which allow you to pay traders with the available currency or even goods payments even if you encounter some loss it will spare you from the effort and the dilemma of acquiring currency to pay your traders in time .... all this of course is based on your amounts of trading if its unlikely small then you should just digest that loss and manipulation from your representative and make a dividend for that ...