What is the ruling on money acquired from previous transactions tainted by delayed payment, lack of immediate possession, and failure to specify the exchange rate?

Possession is a condition for the validity of a currency exchange (sarf). If it is missed, the contract becomes invalid, and it is obligatory to return both exchanged items. If this becomes impossible for some reason, then it is not forbidden for the questioner to benefit from what he acquired from previous transactions, nor is he obliged to dispose of them, unless he knew of the prohibition and intentionally committed the act.