Demon Contract Reward
Quoted from High School DxD novel Volume One.
The job goes like this:
First we are summoned, and then we make a contract with them.
After that, we fulfill their wishes.
As a price, we receive an award fitting for the particular wish they made.
It can be money, an object, and even their life.
Well lately, there aren’t any contractors who would use their life to make a wish. Even if there was a person like that, if the price they gave doesn’t match up to the wish they wish for, then the wish won’t be granted. According to Buchou, “People’s values aren’t equal”.
So what demons gets from a contract differs depending on the contract. They can get anything, depending on the agreement.
As for "level up", I'm not sure what kind of level up you mean, but what has been shown in the story so far is that a demon can get recognized as said by Rias (also from Volume One).
Since this only increases the number of servants, it doesn’t increase the number of powerful devils. So the devils added a new rule. Chances or opportunities were given to powerful reincarnated devils, in other words devils that were reincarnated from humans. Reincarnated ones are given peerage as well if they are powerful. Because of that there are lots of devils in human’s society. There are also devils like myself who came to human’s society as well. Ise, you may not have noticed but you probably passed devils a couple of times while walking.”
The exact mechanic behind it is not clear, but what was clear is that Rating Games is one of the methods to "level up" since it is easy to get recognized if you can proof your strength. This is also the method used by Issei.
As for whether doing contract jobs with humans can get a demon recognized and "level up" it is not clear but it probably is possible though might take much longer time.