Even before Adam (as) was created, Allah told the Angels that He is going to place mankind generations after generations on earth. Some people believe that, had Adam (as) not eaten from the tree then we would all be in Paradise, this is not correct. Allah accepted the repentance of Adam (as), and forgave him before sending him to earth as a Prophet for a time period. Once Adam (as) was on earth, Allah told him that He will continue sending him and his future generations guidance, and those who follow the guidance will not go astray, nor will they be unfortunate.
Allah blessed Adam (as) and Hawa with twins, Qabil(Cain) and his sister followed by another set of twins, Habil (Abel) and his sister. They lived happily on earth. Qabil grew up to be a farmer and Habil a shepherd. Then came time for sons of Adam (as) to get married. At that time children from different pregnancies were allowed to get married, so Qabil was to marry Habil’s sister and Habil was to marry Qabil’s sister. However, Qabil was not happy this arrangement as his sister was more beautiful than Habil’s sister. Qabil became jealous of Habil and refused to obey his father and in doing so he also disobeyed Allah.
Allah then commanded both Qabil and Habil to offer a sacrifice and the ones whose sacrifice is accepted will have the right. Habil offered his best livestock (animal), while Qabil offered his worst crop. Allah accepted the sacrifice from Habil.
We all must understand that Allah has blessed all of us in different ways. We should never look at the blessings of others and feel jealous about it. When giving zakat and helping the poor, we should give that is good and so that it is accepted by Allah.