Hi there, @arkadsolutions.

I appreciate you for reaching out to us here in the Community. I'll help you with recording your transactions with your supplier. 

To get started, you can record the advance payment of £1250 as a check to Accounts Payable. 

Here's how:

1. Open your QBDT file.
2. Go to the Banking page, then click on Write Checks.
3. From the Expenses tab select Accounts Payable on the Account column.
4. Enter the amount, then save.

Then, record the invoice sent by your supplier.

1. Select Enter Bills from the Vendors tab.
2. Fill in the bill details.
3. Click on Save and Close.

Once done, you can now apply the prepayment to the bill.

1. Go to the Vendors menu, then select Pay Bills.
2. Highlight the bill you want to associate with the prepayment.
3. Click on Set Credits.
4. The prepayment will display on the Set Credits screen. Make sure it is checked, then select Done.

You can open this link to learn more about the process: Record supplier prepayments or deposits for prepaid parts or services.

If your supplier gives you a refund cheque for the overpayment amount, you can use this article as your guide in handling the entry: Record a supplier refund in QuickBooks Desktop.

Aside from vendor payments and refunds, you can also review the topics from this link for more hints about tracking other vendor transactions in QBDT: Accounts Payable workflows in QuickBooks Desktop.

If you have any other follow-up questions, please let me know by adding a comment below. I'm more than happy to help. Keep safe!