Manager is a truly capable system in many respects, however, it still leaves much to desire in two areas of interest to the vast majority of businesses, both large and small, namely:
- HR
- Production
I know manager isn’t an HR system, nor is it a production system, however, most other accounting software provide a bit more control over these two areas and it’s become expected.
I don’t specifically advocate for any specific treatment of this particular case, however, any forward movement in making Manager an easier tool for users to Manage all of their financial interactions with employees is welcome. And therefore, I think that this business problem deserves to be solved.
However, I completely agree with @lubos and @Tut on this, in that Payslips are not the right place to track billing of customers. Excuse me @Jason_de_Veyra for my poor choice of words, but this will only create a mess 
I mean @Tut made a good point and @lubos is probably thinking about how to disentagle the situation into a feasible implementation. I know I would.
This gets me to think, that maybe this could be the solution:
@Jason_de_Veyra , wouldn’t it be better if your employees filled up a Timesheet in manager, and these time sheets could be used as a source for both Payslips as well as for Billable Time? This may be the solution that you are looking for and it’s tidier and much more manageable, both in terms of development as well as for users to manage these kinds of situations, and this could be applied accross a variety of situations to link various tabs together like, for example, Projects to both Payslips and Billable Time – as the quoted topic title suggests.