Is zakat obligatory for the past two years on an amount of money that was in another person's possession?

In zakat, the consideration is based on the ownership of the wealth, not its mere possession. If the sales contract with your brother-in-law was concluded and the amount paid became his property as part of the price, then the zakat is due from him. However, if the amount was merely a promise to sell or an advance for it, then it is a debt owed by him, and in this case, the zakat becomes obligatory upon you, either by paying it along with your wealth's zakat when the year (hawl) is complete, or by paying it for past years when you recover the amount.