Verse 20 begins with the words: وَشَرَوْهُ بِثَمَنٍ بَخْسٍ دَرَاهِمَ مَعْدُودَةٍ (And they sold him for a paltry price, for a few silver-coins, and they were disinterested in him). In Arabic, the word: شِرَا (shira' ) is used for buying and selling both. The probability of both meanings exists here. If the pronoun is reverted back to the brothers of Sayyidna Yusuf (علیہ السلام) ، it will mean selling - and if applied to the people of the caravan, it would mean buying. Thus, the sense would be: ` the brothers of Yusuf sold him - or, the people of the caravan bought him - for a paltry price, that is, for a counted few dirhams in return.'
Al-Qurtubi says: The Arab traders used to transact deals involving big amounts by weight while, for amounts not more than forty, they would go by count. Therefore, the word: مَعْدُودَةٍ (ma` dudah: few) used with: دِرَاھُم (darahim: plural of dirham: silver-coins) tells us that the count of dirhams was less than forty. Ibn Kathir, citing the authority of Sayyidna ` Abdullah ibn Masud ؓ ، has written that the deal was closed at twenty dirhams which the ten brothers had divided among themselves at the rate of two dirhams per person. As for the exact amount of dirhams, reported there are other narrations as well which put them as twenty two and forty. (Ibn Kathir)
In the last sentence of the verse: وَكَانُوا فِيهِ مِنَ الزَّاهِدِينَ (and they were disinterested in him), the word: الزَّاهِدِينَ (az-zahidin) is the plural of zahid which is a derivation from zuhd. Literally, zuhd means indifference, disinterestedness. In usage, the lack of interest in and the avoidance of wealth and property in worldly life is referred to as zuhd. The meaning of the verse is that the brothers of Yusuf (علیہ السلام) were really not interested in any financial gain for themselves in this matter. Their real purpose was to separate Sayyidna Yusuf (علیہ السلام) from their father. Therefore, they struck a deal for a very few dirhams.