Imam al-Kisa'i (may Allah have mercy on him) said: 'Iqal' refers to the sadaqah (zakah) of one year. It is narrated from al-Asma'i (may Allah have mercy on him) that it is said: So-and-so was sent to collect the 'iqal' of such-and-such tribe; this is said when he was sent to collect their zakah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الصدقات / 13117