My issue is this

Bzokss

I have a master inventory sheet with hundreds of (part#s) currently in stock with an outbound inventory sheet used via forms. What's the proper formula for when I submit a new form submission on the outbound sheet with (part# & quantity) it will automatically subtract from my master sheet part#s current quantity

Sarah J Tavarez

I'd suggest using a secondary column.
Column A: manually entered Quantity

Column B: =Column A - SUMIF(QTY column on outbound sheet, Part#@cell)