Calculating products in stock

[Greeicy]

Hi guys, I'm new in this forum and I hope you can help me:

I'm doing a small ( Client - Invoice - Products) database management for my small shop.

What I have so far:

I created a Filemaker file with 4 tables:
Clients
Invoices
Product line
Products

On the Invoice table I created a portal, on each portal row I can add a product for each respective invoice. 

The portal row on the invoice table has 4 fields:
SKU number
Category
Quantity 
Price 
Total

Everything works perfect, I can create the invoice, choose a client and add some products, but my question is:

1) How can I make the "Quantity" field on the portal row, to limit the value based on the products i have in stock? let say, If have 3 products in stock, the "Quantity" portal field be limited to choose 1 or 2 or 3 but not more than that.

2) How can I make the "Product" table updates the quantity of the products I have available, based on the products I chose on the invoice?

Any help will be really appreciated!!

:)