Per checking, changing the Quantity in a Partially Fulfilled Transfer Order is currently an Enhancement. It is logged as Enhancement 261716 Transactions > Inventory > Enter Transfer Order list > Edit Fulfilled Transfer Orders > Quantity column field is disabled. Be able to increase/reduce the value in this field even when lines are already Fulfilled.
Here’s a possible alternate solution provided:
Assign an item column field where the user can enter his desired new value for the quantity field.
Create a client script, triggered on field change, that would get the value on the field in Step 1, then set it to the quantity field. Sublist APIs could be used for this case.
Note: This could be used whether the user is to increase or decrease the quantity.
Another quick workaround if needed is that if you only want to correct the Item quantity, you can complete the Transfer Order, and then create an Inventory Adjustment.