@Peanut2019 If you're in the UK & your parents are on a low income or on certain benefits you may be able to get discounted veterinary treatment from the PDSA /blue Cross charities if you have any locally?
It will probably help to give the wound a clean with saline solution as suggested above. But if it's fungal or gets infected it won't cure it & you'll probably need stronger/more expensive medications if it's left to progress into something more significant.
Try & get your parents to have your gorgeous girl treated, she doesn't need an emergency appointment (probably more expensive) but try and get her seen in the next day or two.
Weekend appointments are usually more expensive so one in the week would be preferable.
I think pets at home charge a standard fee whatever day, no more expensive at the weekends.
Would your vets do a payment plan to spread out the cost a little?
Good luck at convincing your parents & as suggested before maybe you could do little jobs or pay with some pocket money to help your parents out? X
Oh & edit, if she's a new shop bought Guinea(had her less than approx 1 month), the shop is responsible for selling 'damaged goods' so you might be able to claim back veterinary costs for certain conditions/fungal infections?
Maybe ask if you can have her treated as part of your Christmas present if you celebrate it?
Xx