What is the ruling if the parties separate after the specific currency has been taken possession of, when that currency turns out to be invalid?

Primary text

If the parties separate after the invalid currency has been taken into possession, the contract is nullified because what was received cannot serve as proper compensation, meaning they separated before the true price was secured. This applies unless the narration permitting the actions of an unauthorized intermediary (*fuduli*) is followed, or if one holds that currency cannot be specified or individualized for the purpose of a contract.