If they void the first contract then there will be a $1 mil dead money hit.
Theoretically they could extend him another year and not give him a new signing bonus but split that 1 mil between both years. So $500k cap hit each year. But of course you would need to give him a base salary as well. So it won't help in a lot of cases.
But what they did with Osi was give him a signing bonus of like $6 mil and gave him an extra voidable year. So Osi has a 3 mil dead cap hit this season despite his contract being voided