How we do it is by using a blended rate.  Here is an example:

1. We pay a 50% deposit to our vendor at 1 exchange rate.

2.  We don't the other 50% until we receive the item so the exchange rate is now different. 

3. So to get the correct exchange rate we use the following formula:

50% deposit payment in foreign currency  times "x" rate = amount in USD

50% balance payment in foreign currency times "x rate = amount in USD

£100 x 1.21 = $121.00

£100 x 1.31 =  $131.00

Total

£200 / $252 = 1.2600 (blended rate)

Use the blended rate as the total paid and it should balance it out.