Suppose gold futures are selling at 360 in February, and 370 in April. Interest is 9% annually. Note that 360*(1+0.09*2/12)=365.4, so the April futures is overpriced. Then we can sell April and buy February. We borrow 360 in February, hold the gold until April, and pay off the loan and deliver gold in April to get $4.6 in April for sure.
I am not sure what the analogous trade is if the April futures is underpriced - say April is 364. Then we should buy April and sell February. But we don't have gold to deliver in February.
Any help completing this last trade is greatly appreciated. Thanks in advance.