Need to pick the available quantity first and then the substitute Item quantity?

We are using the substitute item functionality for Transfer Orders and have noted that when a transfer order is raised and while doing the picking for the same, the system does not validate the existing on hand for the Original Item, while it is picking the substitute Item for the entire quantity.

Example:
On hand for Item A (original Item) - 30
On hand for Item B (Substitute Item) - 100
Requested Quantity - 70
In the above-mentioned case, the expectation is that while doing the picking, the system would pick 30 quantities for Item A and 40 quantities of Item B. But the system is picking the entire 70 quantity for Item B which is a Substitute Item.