nah that's not quite how commodity futures work. when you buy a futures contract, let's say for june 2020, you're saying "at this price i bought it at now, i am betting that oil will sell in june at this price." think of it as buying a barrel of oil that you will sell in the future
if the price of oil is higher than the future you paid for, you make the difference. if the price of oil is lower, you lose the difference
that's a simple explanation, it's a bit more complicated than that. there is some capital investment required, so when the oil was at negative prices you couldn't just promise to deliver all the world's oil in june and reap absurd profit when the price is higher than negative. also because the prices are tied to supply and demand, if you were buying up a shitton of futures contracts for june you are increasing the price of the future contract, thus not getting yield. it was such a low price because there was no demand for it because you would be losing money at any higher price
an example of how you could lose money on those negative prices: if you bought a negative price contract, for june, but hypothetically in june if demand was still so low and supply so high that you can't sell the oil, you promised to sell it at the market price of negative, so whatever you gained of negative price you pay in the sale - and if the price got lower you pay even more than you received. even if bought the barrels yourself and not the contract, you'd have to pay for the storage and transport/disposal of it which would be why it had a negative price. it's really strange and bizarre to wrap your head around, and it's really funny to pretend otherwise, but negative prices do not mean free money
