I'm pretty sure all of this is possible, but I'm having a hard time visualising what yo are trying to do.
Could you give a little more information on what kinds of data you are dealing with?
I assume you have a client list, but what is your product? What do you put on your invoices?
And how does petty cash work?
It sounds to me like you might need some sort of "Balance" type table.
Where both + (owed) and -(payed) amounts get registered. So you can always calculate how much a client had payed and still needs to pay.
But I feel I'm still lacking some information.
If you could tell me some more we might be able to come to some sort of solution.
The White Paper for FMP Novices is useful
Lynda dot com has Filemaker Videos (portions are free)
Free unlocked templates are useful for examining design
Starting Point -
A free calendar is available to be bolted onto your database
You can search specific Filemaker sites on this Custom Google Search
Looking at FMP business database demos is useful - some are fully useable
The Excelisys Business Tracker V3.0