The donation came after Zac's father Mark had estimated the total to be £420,000.
It is the culmination of a huge fundraising campaign that first captured the hearts of Zac's hometown Broseley, then Shropshire, and now the country.
The total means Zac's parents, Hannah and Mark, can now begin the preparations for taking him to Philadelphia where he can undergo CAR-T therapy unavailable on the NHS.
Four-year-old Zac needs the treatment because he suffers from a rare form of leukaemia, called near haploid.
Writing on facebook last night, Zac's mum, Hannah Oliver-Willets said: "We would like to make a VERY special announcement....
"Today we received a very large anonymous donation. This means that as soon as all of the money which people have raised and told us about has been banked, we will be able to show The hospital of Philadelphia and get financial clearance. As soon as we get financial clearance, we can send this to the Embassy and get Zacs visa confirmed. There is around £116,000 outstanding. We urge you all to please please please count and bank what you have.
"We are absolutely amazed at all of your dedication, commitment and hardwork to have got us to this position in such a short amount of time. In fact we honestly can't express just how happy and excited we are right now to be able to say that we WILL be able to get Zac the treatment he needs to have the best chance of survival.
"Unfortunately we are not quite out of the woods yet. We still have a very hard and tiresome road ahead of us over the next 4 months. But with any luck we will be able to celebrate in about 4 months time after Zac has had his car-t therapy, that you all made possible. Please can you all raise a glass tonight as tonight marks the days when we can all smile and breath for a moment in piece and happiness.
"Now we have reached £500,000 (including everything that's not quite banked yet). We can safely say that Zac can have his treatment and we can afford part of our accommodation and food. We do however, need to return to the US every 3 months for a year and every 6 months for 2 years for bone marrow biopsies. We haven't worked out how we will pay for this.
"So we would like to ask you all if you could please to continue to fund raise to ensure we can successfully finish Zacs treatment. We also found out today that Zacs night feed and out patient medication will cost over £1000 a month whilst we are out there. So if we can just keep going until Christmas at least we will know that we are on safe ground.
And as for the mystery donor. Well, I can tell you all, that is such an interesting story, but I've waffled on far too long so will have to tell you all about it at another time.
Much love and hugs xxxx"
How you can continue to help:
Anyone who wants to support the fundraising campaign should visit justgiving.com/crowdfunding/zac-oliver
People can also join Zac's Facebook support page which is called 'Zachariah's's fight against Acute Lymphoblastic Leukaemia'.
People can also donate using their mobile phone by texting ZACH75 £1 to 70070.