Vodacom airtime advance given by force

Y is vodacom giving me airtime advance by force? I attempted on exchanging my airtime fr their minutes only to find dat i dnt hv inaf airtime,instead of vodacom declining my request they gv me airtime advance which il b charfed extra R1.00 wen i return it,i do not nid their advance i hv money to buy airtime juz i wasnt aware that i hv less airtime,pls vodacom reverse ur advance i dnt nid it and y automatically giving me ur advance and yet charging fr extea R1.00,m new to voda and m not happy with wat they doing to me.