Payment received as petty cash

Recording payments as petty cash in QuickBooks Online is easy, @DougPLX X.

I suggest setting up an account named Petty Cash on your Chart of accounts.

Follow this steps on how you can record your petty cash transaction:
 

1. Go to Settings  and select Chart of accounts.
2. Select New. Depending on your account view, you'll either see a panel labeled New account or New category.
3. Select the option you see below for next steps.

Once done, manually transfer petty cash to the bank linked on your QuickBooks account.

Here's how:
 

1. Select + New.
2. Select Transfer.
3. From the Transfer Funds From dropdown list, select the bank account that the money is coming from.
4. From the Transfer Funds To dropdown list, select the bank account that the money is going to.
5. In the Transfer Amount field, enter the amount being transferred.
6. Edit the Date field.
7. Enter a description of the transfer in the Memo field (optional).
8. Select Save and close or Save and new to complete the transfer.
9. Select Yes if you double checked the date.

However, I still suggest consulting an accountant to guide you in choosing the right accounts for accuracy.

In addition, here's an article that guides you on how to record the most frequently used transactions: Record commonly-used transactions.

If you have further questions about recording petty cash in QuickBooks, feel free to reply below. I am always here to help. Have a great day!

@DougPLX 

"I want to record this amount received in quickbooks without it being classed as income"

Create an other current liability account called 'Customer Advances' or 'Customer Deposits' and assign that to the advance received.

"The payments I am making for the purchases are from the bank account linked to my quickbooks, so show up as expenses - how do I then link these to the money I have received?"

From an accounting perspective there is no connection between the cash advance and the expenses.  The advance is an increase in cash and the expenses are a reduction in cash.  The advance will be converted to income when you invoice the customer.  For tracking purposes, assign the advance and the expenses to the same customer/sub-customer and that's as good as you can do.