QB Issue Resolution:

Run  a quick report on the assembly item, set the date range to ALL.  For the date of this build, was the assembly item showing a negative qty on hand?

One reason this can happen is that the assembly item was negative on hand.  When that happens QB still posts the assembly item average cost to COGS with the sale.  When you build the assembly item after that, QB looks at the past sale and compares the average cost posted to the current build average cost, and makes an adjusting entry to COGS and inventory asset.

Also on the assembly item use the scroll bar for the BOM section and insure that there is not another item listed below the display – rare but it happens sometimes when people edit and delete lines.

Resolution for Issue 'Building assemblies creating a negative (credit) balance' available: Yes (Solved).
Source: Intuit Community forum.

Building assemblies creating a negative (credit) balance: 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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