If you have an arrangement for your mum to pay half, that should not
be considered in how you will pay the retail price at an Apple Store or
through a custom order online to be delivered to an Apple Store or an
authorized Apple reseller (independent) business.
To get a computer shipped to you, or to the store for you from Apple
that is a custom order, the page(s) I cited say that you are to pay for
the item before it would be shipped. Per the above post, note:
Re: Can I purchase a customized macbook pro and pay for it in store?
Choose your country or region - Apple:
https://www.apple.com/choose-your-country/
Apple - Find Locations - Country Selector - main:
https://locate.apple.com/
Find an Apple Authorized Service Provider
Make an appointment at an Apple Retail Store Genius Bar
Contacting Apple:
https://www.apple.com/contact/
Sales Policies - Apple:
http://www.apple.com/shop/browse/open/salespolicies
Whether or not you pay or your mum pays, these reseller outlets usually
expect to get a full amount when due; so to be sure we aren't mixing up
more than one question. Anyway, ask the official sources... If you are in
the USA or where Official Apple retail Stores exist, and also, where an
online Official Apple store exists; then you may talk to both to see what
if anything they can work out for you. The policies for Apple Inc are in
their web site. You may have other rights depending on where you are,
but their delivery and sales are fairly standard; & I copy-pasted from the
official Apple site into my first reply.
An official Apple retail Store may be able to get the build model you seek
and perhaps (ask manager) if deposit is required for them to take an order
where it'd go to their Store; where you could pick up &/or pay balance due.
The usual path, indicated in their printed/online info, is you pay in advance.
Good luck & happy computing! 🙂