Received commission in 2018, returned commission in 2019

I received a large commission in 2018. I reported it as income on my tax return. The client cancelled the policy in 2019. It is setup that the insurance company does not pay me until I pay off the owed  commission. The new commissions I earned in 2019 reduces the liability of the owed commission. I won't be able to pay off by end of 2019. How should I file my 2019 tax return?