Sunnah.com reference : Book 7, Hadith 23English translation : Book 7, Hadith 0Arabic reference : Book 7, Hadith 799
Abu Dawud has:
"If anyone makes to transactions combined in one he is entitled to take the lower price, or (it is considered) usury."
وَلِأَبِي دَاوُدَ: { مَنْ بَاعَ بَيْعَتَيْنِ فِي بَيْعَةٍ فَلَهُ أَوَكَسُهُمَا, أَوْ اَلرِّبَا }   1‏ .‏‏1 ‏- حسن.‏ رواه أبو داود ( 3460 )‏.‏