Every now and then, your missionary might find himself short on cash. Though the best policy is to let the missionary work out his own finances, there are times when emergencies arise and having a little extra help from home is nice.
One helpful suggestion: It is very easy to access emergency cash with an ATM card in the mission and cheaper and quicker than using the bank transfers that have been suggested in the past. You just keep some money in your missionary's account. When unexpected expenses arise, he can withdraw as needed, and you keep track by checking the monthly statement. You also get a better exchange rate through ATMs because they use the big bank commercial rate.
As of the time of writing (March 2006), the Euro is currently worth about $1.19. So, if you deposit $100 in your missionary's account, it'll be worth about 84€.