I am not sure if all connection carriers are the same but for delta imputed income is based on yield fare, so your domestic partner would get the yield fare added into their taxable income and depending on their tax bracket their check would be this much less. But, not sure if I am correct as a yield fare to FLL is approx $60 and if they are in a 30% tax bracket they should only have approx $18 withheld from their paycheck.

Does your domestic partner's company have a travel manual? If so it should state how the imputed income works.

Tony