If you want to know if there is a significant difference in the "before" and "after", then you probably should do a Paired t-test. These tests are done when the same individuals are measured twice.
When you perform the Paired t-test, you choose the significance level that you want and then you check the p-value obtained from the test. If the p-value is lower than the significance level you choose, then you can reject the null hypothesis that the "before" remuneration and "after" remuneration values are the same, i.e, there is a significant difference. However, I don't understand what is your question regarding the sample.
Edit: If you want to test a "group" vs another different "group" then you can just do a regular unpaired t-test. It handles unequal sample sizes, however, you have to check if the assumptions for the test hold. If you are also concerned about unequal variance (or standard deviation), then you should use a slightly modified version of the unpaired t-test called the Welch test.
Do you have the pay for each one in the 300 employees group and the pay for each of the 400 employees group? Or do you only know the total (30k and 32k)? If you only have the total, I don't think you can do much.