Although texts such as [Berg *et al.*](https://www.ncbi.nlm.nih.gov/books/NBK22555/#A211) tend to refer to inorganic phosphate, $\ce{P_i}$,  as [orthophosphate ($\ce{PO4^{3-}}$)](https://en.wikipedia.org/wiki/Phosphoric_acids_and_phosphates#Orthophosphate), the term inorganic phosphate is used because in aqueous solution at pH 7.6 several phosphate species exist, the predominant one being [$\ce{HPO4^{2-}}$](https://en.wikipedia.org/wiki/Phosphate). 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](https://www.amazon.co.uk/Structure-Mechanism-Protein-Science-Catalysis/dp/0716732688) actually writes the equation of the reaction (16-1) with $\ce{HPO4^{2-}}$ rather than $\ce{P_i}$.