Acharai Mos

The Torah expounds upon various fundamental laws in our parsha after which it says ושמרתם את חקתי ואת משפטי , you shall observe My decrees and my judgments, אשר יעשה אתם האדם וחי בהם אני ה , which man shall carry out and live by them - I am Hashem. 

Rashi explains that וחי בהם means in the world to come where there is eternal life & 'אני ה, I am Hashem who is faithful to pay reward.

The gemara explains that the reward for mitzvos isn't possible to receive on this world, for that's how great the reward is.  It's an eternal reward that we would only be able to appreciate in an eternal world. However, this   contradicts another gemara that says that Hashem pays back the ones who are hated in this world so that they don't merit the next world. So, we see that there is reward for mitzvos in this world.  Even the reshaim have some mitzvos for which Hashem pays them back on this world, so is there or isn't there reward for mitzvos on this world?

Rav Shmuel Wallkin says in his sefer, that he heard from the Chafetz Chaim how Hashem calculates the reward a person gets.  Really each person determines his own reward.  If a person has a set time for learning or davening & he's on the way & precisely at that moment a big deal comes up in which he can make a lot of money but he doesn't give in to temptation, then that person showed how valuable his learning is & his reward will be that great. However, if when he was on his way to shul & something came up where he would just make a few dollars & still he stopped going to shul to make a measly few bucks, that shows how little value his mitzvos have for him & he will be paid accordingly. 

There's a story told of a person who needed a large sum of money to make a wedding. He went to his rabbi for a bracha which the rabbi gave him. The rabbi told him that he should take the first deal he's offered when he leaves, for that deal will be his blessing. When he left and made his way home, he stopped at an inn where nearby were a group of merchants doing business. When they saw this man coming close to them, one of them thought it would be fun to play around with him & asked him, “hey, do you want to do some business with me?”

Remembering what his rabbi said, he answered yes right away. The merchant asked him, “are you willing to buy my olam habba for a ruble?” The man answered yes. They made the deal, had witnesses sign the document & concluded a real sale. The merchant thought it was all a big joke while the man took the sale very seriously. 

When the merchant got home, he told his wife what happened to him at the inn & how that man bought his olam habba. He was expecting her to laugh along but she got very upset & said, “how could you do that? I don't want to live with a man who would sell his olam habba; go buy it back. 

Left with no choice, the merchant went to find the man to buy back his olam habba. However, the man wouldn't sell it. He tried offering more money but the man wouldn't sell. He couldn't go home, so left with no choice he went to find this man's rabbi to ask him to intervene so he could get his olam habba back. When he told the rabbi what happened, he said my student is right. However I can get him to give it back to you on one condition. He is making a wedding. If you will cover all the expenses of the wedding, I will get him to give it back to you. So the man agreed & the deal was done. Afterward, the merchant turns to the rabbi & says, “I did what you told me but where is the justice? Yesterday this man paid one ruble for the merchandise & now I'm paying thousands for the same merchandise?” 

The rabbi answered that the price of merchandise is based on its value. Yesterday you belittled the value of your olam habba & sold it for what it was worth to you - a ruble. Now, today that your marriage hinges on this same merchandise, you have to buy it for what it's worth to you now, thousands.

One determines his own value.  Maybe that's why Hashem is able to reward the sinners for their few mitzvos on this world, for if the value is determined by them & they don't really put too much value on the mitzvos, then the reward isn't that great either. It could be given on this world. However, for the people that are doing the mitzvos because they want to keep Hashem’s commands & try to do the best they can, their reward is eternal & can't be given on this world. Let's remember this when we do the mitzvos, that we are the ones who determine our reward!