QB Issue Resolution:

When you prepay for the items, on the payment select accounts payable as the expense – that will set a vendor credit (deposit) just like what the vendors does for you in his books

when the items come in, enter the bill, use pay bills, apply the credit, pay the balance if any, save

the item will stock as of the bill date

QB does allow for pending bills, in QB a PO serves that purpose

Resolution for Issue 'why is inventory increased when a bill is created vs the bill date?' available: Yes (Solved).
Source: Intuit Community forum.

why is inventory increased when a bill is created vs the bill date?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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