I want vlookup gets the value once
 

I do not know how to express myself with my poor english but I have a
case
that vlookup getting value from another sheet which takes values from
Internet (euro/usd rate). But I want vlookup gets the value once and
not each
time that workbook is opened. I am getting always the same rate for all

records and I do not want this .. How to solve ? Please h

Sheet1 :
A1 A2 A3
RATE
01.Jan EUR 1,7
A3-- Formula:=VLOOKUP(A2;RATES!$E$10:$F$12;2;FALSE)
02.Jan EUR 1,7
03.Jan EUR 1,7
26.Jan EUR 1,7

Rate in Sheet RATES! is changing daily.
I want that old dates shows reciprocal values and not the latest one.
Many thanks