> Offering that sponsorship has very real costs to the company, well into the thousands of dollars per year. It seems perfectly reasonable to me for the employer to deduct that cost relative to what another employee who doesn’t bring those additional costs.

Let me fix that for you:

"Purchasing that slave has very real costs to the company, well into the thousands of dollars per year. It seems perfectly reasonable to me for the owner to deduct that cost relative to what another freeman who doesn’t bring those additional costs."

Remember the immigrant suffers additional liabilities and risks a citizen wouldn't. The employer in this case is in a much more powerful position to keep the employee in line.