Nested IF formula that can rewrite another cell

I have no idea whether this will end up being a formula or a Macro, but any help would be appreciated:
I have an inventory sheet and am attempting to create a running log. The idea being that there are 2 columns: On hand Inventory [A]; and Deduction [B].
The idea is that A changes based off the value of B but retain the new value, and B will reset itself to 0. So, if Someone removes something from Inventory, They put the number they are removing in B and A will update itself to subtract whatever the value is, and B will reset itself to 0.

I think of this as a looped formula, but I've found that's not the term for what I'm hoping to achieve. Can anyone think of a possible solution to this?

Thanks for helping.