Here the "retenue" is referring to an amount. In any financial/taxation/administrative procedure, an amount can be "retenu" following a series of calculations and in that case, the "montant retenu" means the amount that has been arrived at following a number of calculations. That is the case here :
- part salariale...retenue = the amount of the employee's contributions;
- part patronale...retenue = the amount of the employer's contributions.
There are all sorts of circumstances in which an amount can be calculated to be either positive or negative. Both colums figure in the employer's column in your source. It is sufficient to use the term amount in both cases, the (+) and (-).

Example: the employer may have to pay a certain percentage of the amount of the employee's basic salary by way of health fund contributions. However, the employer may be entitled to a certain reduction in that amount, if he has set up his business recently, recruited a young person, a person who had been unemployed for a long time, has set up his business in an area classified as having a particularly high rate of unemployment, etc. Those are the sort of factors that are likely to be accounted for in the (-) column.

Bear in mind that "retenue" as a noun is not always referring to an amount that has been withheld, but that it can also be used to describe the amount that is being used officially, i.e.: the amount arrived at, thus "amount" tout court!