Best way to bill a tenant for fuel oil?

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I'm a complete newbie to landlording. I just rented out half of my two unit two months ago (I'm living in the other unit).  The units are on separate utilities, each with its own fuel oil furnace. I just replaced my furnace and in doing so I switched to natural gas since it will be a lot cheaper to run.

Now that I've done that I have a couple hundred gallons of oil sitting in my basement not hooked up to a furnace.  I was thinking I could pump it over to the other unit and fill that tank up pretty easily. Now the question I have is, what's the proper way to charge the tenant for that oil?

Eventually the oil supplier will come periodically, fill up the tank, charge the tenant directly, and I'll be out of the picture, but since I've already paid for the oil in my tank I would want to charge the tenant for what he's using.

Any advice on the subject would be appreciated.