A lease rate can be a + or a - to the forward price, depending on the magnitude of the two components of a lease rate: “storage costs” and “convenience yield”. Storage costs push the forward price in the direction of contango (a higher forward price than the spot price). Storage costs are costs to the holder of the commodity. So for high storage costs the forward price would be driven up to compensate the holder. Convenience yield is the price the holder of the commodity is willing to pay for the convenience of holding the commodity (think a commercial user that needs constant access). A high lease rate (higher than the risk-free rate) would lead to backwardation, where the forward price is less than the spot price. A lease rate that is lower than the risk free rate would result in contango.
“The lease rate on a commodity is the interest rate (ie return) the holder of a commodity would require to lend it out, and it is analogous to the dividend yield on a stock that has been loaned out for a short sale.” - Schweser So the holder can require a positive return if there is a “convenience yield” (similar to a divedend), or the holder can require a negative return if there are “storage costs”. The lease rate would be the net of those two aspects.