I will bless those who bless you, and those who curse you I will curse. (12: 2)

From a simple reading of the words it seems that those who bless Avram will be blessed by G-d earlier, before they bless him, but those who curse him will only be cursed after they have done so. This is the meaning of the words “I will bless (previously) those who bless you, and those who curse you I will curse (afterwards). To be parallel the verse should have said “I will curse those who curse you”. This requires explanation:

The intention of the verse may be that the blessing of a rich man is more valuable than the blessing of a pauper. Conversely the curse of a poor man is worse than the curse of a wealthy person.

Because a rich person is stuffed full of everything good, when he blesses he blesses with plenty of goodness, since just having average wealth is considered ‘normal’ for him, therefore it would not be considered by him to be a blessing. So when he blesses with goodness his blessing is expansive and beyond the average wealth. For a pauper it is the opposite, since his soul is empty from the goodness of life, and he lives on a crust of bread, his concept of blessing is stunted and limited, and he would consider even a meagre existence to be a blessing.

Therefore G-d wanted that those who bless Avraham should be able to conceive of a generous blessing. That is why G-d gave them blessing beforehand, so that when they come to bless Avraham they will bless with plenty. So the verse says “I will bless those who bless you”.

With a curse it is the opposite. Someone who is stuffed full of goodness, when he curses he thinks of someone who has meat and wine, but not of such good quality. However a poor person, since he knows starvation, curses with famine and thirst.

When G-d wants that someone who curses Avraham will themselves be cursed, He wants them first to curse while they still have wealth and only afterwards to be cursed themselves. So the verse says “Those who curse you I will curse”, they will only be cursed after they have cursed Avraham, in order to limit the severity of their curse.
This explains the order of the words in the verse.