It is not a big problem. The distance involved is too short. The TTE may not turn up till the train leaves Borivili. But better honest way is to buy open tickets from Bandra to Borivili before boarding the train and offer to pay the difference in fare between the sitting class and 2A class.
Or it can be explained to the TTE and if demanded, the fare from Bandra to Borivili can be paid in cash and a receipt taken from TTE.