How do you split the proceeds from a home sale when the parties contributed an unequal amount to the down payment?

The question

My husband and I purchased a home with my mother-in-law and it isn’t working out so we want to sell. Our portion of the down payment was $80k and her portion was $8k. I read that splitting the proceeds is based on a percentage of the amount paid at closing by each party. Is this true? A split of 50/50 definitely does not seem fair.