I don't think there is any way to force Sage 50 to do this, sorry, without adjusting all your current 300 inventory out, receiving the 500 new inventory, selling the new 500 inventory, then adjusting the current 300 inventory back in again.

This may be just my opinion but if your business is truly project-based, you would not have any inventory in stock, so it would not be an issue. However, you do have inventory and decided at some point it should be FIFO-based so it is doing what you asked of it.

If you use Sage 50 Quantum, and use Locations (warehouses), you can use a separate 'warehouse' for Projects.

Sage 50 Quantum Edition, formerly known as 'Simply Accounting Enterprise Edition') keeps separate inventory cost ledgers for each 'Location'.

(I wouldn't use more than a couple dozen warehouse / Locations in Sage 50, definitely don't set up a separate Location for each Project unless you only do a few each year, close them out at the end, then mark them 'inactive'.)

As Richard said, the accounting is technically correct. Another work-around is to use a different item code in Sage 50 - i.e. 'Widgetx500' to track that specific purchase. It may give you better / more relevant information on your reports.