Although texts such as Berg _et al._ tend to refer to inorganic phosphate, $\ce{P_i}$, as orthophosphate ($\ce{PO4^{3-}}$), the term inorganic phosphate is used because in aqueous solution at pH 7.6 several phosphate species exist, the predominant one being $\ce{HPO4^{2-}}$. If this is regarded as $\ce{P_i}$, then _it_ is the source of the $\ce{H^{+}}$, and the equation balances.
Note added by David:
On checking I find, in contrast to Berg, Lehninger’s book defines $\ce{P_i}$ as monohydrogen phosphate ($\ce{HPO4^{2-}}$), and Fersht actually writes the equation of the reaction (16-1) with $\ce{HPO4^{2-}}$ rather than $\ce{P_i}$.