Zu genehmigende Projektbuchungen
is correct as it is. It is a present participle ("genehmigend") combined with "zu" (this combination expresses that something needs to be done), and used like an adjective (which is why it is inflected according to the following noun, "Projektbuchungen", by adding an "e").
More information and examples for this are available e.g. here.
As mentioned above, this construction indicates that something needs to be done. However, items to approve and items to be approved cannot quite be distinguished like this (and I am not sure what the semantic difference would be).
As pointed out in the linked article, this can be used only in an attributive fashion ("zu genehmigende Projektbuchungen"), not as "
Projektbuchungen sind zu genehmigend". In the latter case, the infinitive with "zu" replaces the present participle with "zu": "Projektbuchungen sind zu genehmigen".
Zu genehmigen Projektbuchungen
would be incorrect; it would not be a grammatically correct construction. With an additional colon, it might be transformed into a somewhat unelegant alternative way of expressing "Projektbuchungen sind zu genehmigen":
Zu genehmigen: Projektbuchungen
This would only make sense if there were several statuses, though (such as "zu genehmigen"), and several items that are affected by one of those statuses, as in:
zu genehmigen: Projektbuchungen, Kaffeekontingent
erledigt: Begrüßung, Überweisung der Gebühren, Gefangenenaustausch
Projektbuchungen zu genehmigen
could be correct, but usually only in the context of a longer sentence. For example:
Es sind noch mehrere Projektbuchungen zu genehmigen.