Negative prices for energy are normal and we pass them through the same way as positive energy prices. With negative energy prices, the effect on the producers and consumers of energy are reversed: generators pay out cash for their exported energy, while consumers receive cash for their imported energy.
In the wholesale spot energy market, prices become negative when there is excess generation in the system.
Persons with generators in the Localvolts market can also offer their energy at negative prices to transfer cash to the targeted consumer(s).