It is not common to find a hydraulic portion of the ABS system leaking; however, it happens.
When the hydraulic block is leaking, the whole assembly is changed as a unit.

The area must be throughly cleaned and the leak must be pinpointed before the unit is blamed as the problem. I remember reading somewhere about tightening the screws of the block to either slow or stop the leak, this may or may not be your case.

The mechanic's preference is not to use used parts, if the replacement part is defective, then he will have to charge you again for the labor to remove and replace, a used part could introduce contamination into the system, maybe he cares too much about the work he does, maybe he will not be getting the profit of selling you a new part, etc. etc.
Anything goes wrong and you and the mechanic will not see eye to eye anymore, it happens all the time, Murphy cursed the automotive repair industry since the day he was born.
My preference and personal style is to do a good job the first time, re-working on something is way to expensive and relatioships and trust suffer immediately.