Qty availability by option values in Zen Cart

A garment company has selected Zen Cart as shopping cart for its online ecommerce. Each dress model in the catalog has a size attribute. Each size attribute has a list of option values representing the actual size numbers available for that model. The problem with the current Zen Cart (version 1.2.3d) is that it's not possible to enter the available quantity for each size value but globally only, that is not considering size numbers at all. In other words you can know that a certain number of garments is in stock but you can not know if, for a given size number, a certain garment has been sold out or not. We would to have qty availability for each option value so that the global qty will be the sum of each option value quantity. Can you modify the last source version and add that feature? Note that this must be controlled by the admin panel as usual and that, as natural, any quantity has to be correctly managed by the cart system. You will provide installation instruction in order to integrate modifications into the last Zen Cart version available. My timeline require that you provide that within 10 days after the end of this announce. We require that you have experience with Zen Cart and that you have already have understood by that description what to do in order to accomplish the goal. We offer 120USD for that work. Since we need other features in the next month a good work will probably result in one or more other Zen Cart customizations. Thank you. Best regards.