What the best way to design an increase/decrease field?

I'm working on a simple settings panel for an ecommerce site for adjusting a product's inventory quantity in a trackable manner. My team is in disagreement in regards to the most efficient way to design a form to adjust the quantity. We're currently in two camps.

First Option (Manual)

A user chooses "Increase" or "Decrease", then enters a value that will be used absolutely

Second Option (Automatic)

A user enters a positive or negative number (using a minus sign), then receives confirmation via a small badge

We want our users to easily understand what kinds of values the Amount field accepts, but we also want it to be as efficient as possible.

The Manual option is clearer, since the user knows whether addition or subtraction will be performed.

On the other hand, the Automatic is slightly less clearer on what sort of values are acceptable, but becomes faster to use once the user understands how the form works.

What's the best way to design this?