وَمَنْ بَاعَ سِلْعَةً بِسِلْعَةٍ أَوْ ثَمَنًا بِثَمَنٍ قِيلَ لَهُمَا: سَلِّمَا مَعًا
Translation: Whoever exchanges one commodity for another, or one monetary consideration for another, both parties are told to deliver simultaneously.
Silah bi-silah means one commodity exchanged for another, such as one particular book for another. Thamanan bi-thaman concerns price or currency exchanged for price or currency.
Quduri directs both parties to deliver together, with neither side generally postponed as in the simple cash-for-goods sequence.
Currency exchanges also carry the separate conditions of the law of sarf, which this sentence does not remove.