There is no problem at all! You only have to make the right calculation as the MW is not the same (basically the dodecahydrate exhibits an heavier MW due to the presence of 12 water molecules per molecule of disodiumphosphate) ... The other issue is to be careful when dissolving the anhydrous salt into water (this is a highly exothermic reaction).
I agree with true answer of Dr. Dominique Liger (Université Paris-Sud 11, Institut de Biochimie et de Biophysique Moléculaire et Cellulaire (IBBMC) Orsay, Île-de-France, France) at all.