(CS2103T-T11-03) Minor suggestion regarding finance commands.

[yan-soon]

I really like the update_inventory command as sometimes when I run into some faulty items such that I have to subtract them off, the update_inventory allows me to simply subtract the number of faulty items, instead of having to do the actual subtraction myself.

I think the same can be implemented for the finance portion as when I run into issues when I have to minus/add to my costs, I can simply update it without having to do the extra calculation myself.