Comparison of two configurations to balance source impedance
The sensor is floating wrt Diff ended amplifer's AI GND


If it is truly floating and presents a high impedance to ground then there is nothing wrong with scenario 1 (buffer amp not required) because the 1 kohm can be regarded as being two 500 ohm resistors; one in each leg. If you regard the source as a black box then you cannot differentiate between one leg having 1 k, the other leg having 1 k or both having 500 ohms.

However, the devil is in the detail and nothing is as simple as this scenario BUT without further information then that is all that can be said.