You could use either MATCH( ) or VLOOKUP( ) to see if the ProductID in
the Orders sheet exists in column D of the Assetts sheet.

An INDEX( ) function around the MATCH( ) function, or just a VLOOKUP(
), could be used to return the quantity in the Assetts sheet, to ensure
that Quantity Rec in the Orders sheet does not exceed it.

A function can only pull data from somewhere else - it can't "push" it,
so you would need a different approach to achieve your point 3.

Hope this helps.

Pete