Shulchan Aruch, Orach Chayyim 191 (Modified)
Laborers, who are engaged with work at an employer, should abbreviate the Grace After Meals (Birkat Hamazon in Hebrew) in order not to waste the work time of the employer…
In what situations does the previous ruling (about abbreviating the Grace) apply? In a situation where they get paid for their work above and beyond the meal itself. However, if they only get paid with the meal itself that they eat, then they should recite the full 4 blessings of the Grace After Meals as they were arranged (i.e., in full). Furthermore, if the employer eats with them, then even if they do get paid beyond the meal itself, they should recite the full 4 blessings.
Source: English Translation by Jon Levisohn. Hebrew text from Sefaria.org. Interpretation found in Rabbi Jill Jacobs’ “Jewish Employee-Employer Relations.” MyJewishLearning.com.