Procedure to Define Both Currency USD and EURO

I am new to SAP MM. Pls help me. I have one question regarding vendor in purchasing.
We are purchasing export item, export item currency is USD. Some time same vendor send item with different currency like EURO. I'm facing problem when we pay to vendor.
In my Vendor Master Currency is defined as USD. I can pay in USD.
If I want to pay in EURO, again I have to go vendor master and change USD to Euro vice versa.
Everytime I am changing.
Is there any procedure to define both currency USD and EURO.
According to situation I can select currency.
My local currency is different i.e SAR (Saudi Riyal).

This is a common problem in almost all factories in our group. To simply the matters let me explain the method we use with some numbers.

1) Lets say we raise a PO as follows:
2 items @ $10 = $ 20

2) GRN is raised as per PO

3) Invoice comes from the vendor as
2 items @ Euro 0.80 = Euro 1.60

4) What we do is that we check with our purchasing people whether the Euro to $ convertion from the supplier is beneficial to us, if not it is within our tolerance levels.
Then we process the payment for $ but mention the correct EURO amount in narration. Bank instructions goes for EURO and there will be a Exchange diff entry to the bank rec at the month end.

5) As far as I know you can't maintain multi currency in the vendor master.

6) Another effective method we are adopting is to have two vendor accounts for different currencies.