My advice is that 'allocating stock' is not normally a problem. Everyone (read 'every order') is very happy to grab stock. It is the management of stock UN-allocation that becomes the traditional problem.
An order that had stock allocated to it is shipped. Easy: unallocate the quantity.
An order that had stock allocated to it is cancelled. Easy: unallocate the stock.
An order that had stock allocated to it has its delivery date moved out. Hmm: now what? If it's only a day or two, probably leave as-is. If it is moved out 3 months what do you do? Do you let it hog the stock and stop other urgent orders getting it instead - after all, you could give the stock to the more urgent orders, and by the time the original order needs to be shipped you will have more stock in? Do you unallocate it, and release the stock? Do you unallocate it now, then re-allocate it later? When later? How do you remember? Do you automatically unallocate stock as soon as an order is modified?
What happens if someone orders 100, you check the stock, and there is enough. You allocate the stock to the order. Then they change the order to 600. Do you auto-update the 100 to 600? Maybe you don't have 600. Maybe you have, but you would not want to give that order all of the stock.
What happens if the order is for 600, then it gets reduced to 100? Do you then review other existing orders and potentially re-allocate the 500 now freed up?
What happens if you have an order for 100, and you only have 10 in stock, but the order is due in 3 weeks time, and by then you should have 250 in stock. Are you going to allocate 'notional' stock?
We usually find a logical first step is to display to the users the balance of stock available as they enter each order (just the total stock - all current open orders).
Then we extend it to show a schedule (daily or weekly) what the running balance will be: current stock - current orders + planned purchases.
By the time they have got used to seeing that and speculating what they would have done had a stock allocation system been in place, they have started to see the down-sides and started to devise some policies and procedures.
Yes: I recognise I didn't mention tables, fields, files, or Filemaker in any of that.