how to discount invoice items on inventory table? ( file attached)

Hello , i have 2 tables one inventory and one invoices . 

i would like to know how get on the inventory table the new totals after updating with the invoice table ? 

for example :  i create a new product on inventory  REF: 2020  with quantities by size SIZE1: 10   SIZE2: 10  SIZE3: 10          TOTAL: 30

now i go to invoice and create a new invoice selling REF: 2020   SIZE1: 2      SIZE2: 2      SIZE3: 2       TOTAL SALE: 6

now i want a field on the inventory table that discount those 6 pcs from the total of 30 .... telling me that i only have 24 pcs in stock .

here i attached you a sample file .

i really appreciate you help , 

Thank you very much 

inventoryinc.fmp12.zip