Can the purchasing Unit of Measure be different than the item master unit of measure?

 Can the purchasing Unit of Measure be different than the item master unit of measure?

 We have a part that we stock in ml, however the vendor only sells it in 5 gallon containers. We are trying to figure out how to get our Purchase order and receiver to match the vendor packing list and invoice.

 The part is already set up as ml but the vendor cannot sell it to us in any other unit of measure than 5 gallon container (their each).

What is the best way to go about? Any advice would be very helpful.