I am a QB desktop user and I've been looking at QB Online.  I own restaurants so most of my entries in QB are journal entries.  That's how I get my daily sales, deposits, comps, etc. in QB.

In the situation you reference, on the desktop version, I have "petty cash disbursement" setup as a vendor.  If we should have had a $200 deposit but $100 was spent on an expense instore (like say pest control), I enter the cash deposit as $100 and I have a cash account called petty cash that the other $100 goes in.  I then create an invoice payable to my "petty cash disbursement" vendor for $100, I make sure I note what the expense was for in the memo line (pest control), then I go to pay bills and I pay this petty cash disbursement bill from the petty cash account.  That way money is deposited in the petty cash account from your sales entry, then withdrawn when you pay the petty cash bill.

This has worked well for me over the years but the online version of Quickbooks does not appear to be really focused on accounting (there is a disclaimer "be careful using journal entries, these should only be used minimally and by accounting professionals") but journal entries, invoice entry/payment is almost my entire accounting so...I think I'm sticking with the desktop until I find another accounting solution.  Quickbooks seems to be moving away from accounting and directing you toward pre-determined methods of accounting for your assets, liabilities and cash flow.  And those methods seems to be ripe with room for error.  Very little ability to customize/personalize based upon your business type.

Hope that helps.