It can be, but not necessarily. I'd imagine it would only happen if you had already used the money, and the only way you could pay them back was by foregoing small chunks of your wages every paycheck. A similar thing happened at my company when they overpaid certain employees except they did not "request" it back, nor were the amounts small. In any case, OP is dealing with an optional request, and as such is not required to fulfill it. So if their wages are garnished, I would interpret that as retaliation/wage theft since they have no basis to take their hard earned money over a "request." Not an expert on employment law so take my comment as you will, but in my experience OP wouldn't and shouldn't have to pay anything, and that this letter is sketch as hell.