How do I pay a bill using prepayment account? For instance, I've made $300 prepayment to MrA. A bill to pay MrA is $30 this month. How do I link this two?

Let me help you pay a bill using a prepayment account, @hazel-hlaing-orb. 

You can write a check to the vendor and record it to your Accounts Payable (A/P) account, decreasing the balance until you are ready to enter the final bill.

1. Create a check for the vendor.
   1. Go to the Banking menu, then select Write Checks.
   2. Enter the vendor name, date, and payment amount.
   3. Go to the Expenses tab. In the Account column, enter Accounts Payable.
   4. In the Customer:Job column, select the drop-down, then choose the vendor name.
   5. Press Save & Close.

Then, enter the bill:

1. Go to the Vendors menu, then select Enter Bills or Receive Items (if you like to record the bill later).
2. Fill in the necessary information.
3. Press Save & Close.

Once done, apply the prepayment to a bill by following the steps below: 

1. Go to the Vendors menu, then select Pay Bills.
2. Highlight the bill you want to associate with the prepayment.
3. Select Set Credits.
4. The prepayment will display on the Set Credits screen. Make sure it is checked, then select Done. If there is a balance due on the bill, QuickBooks will create a Bill Payment Check.

To view existing prepayments, just follow the steps below:

1. Go to the Reports menu, then select Vendors & Payables, and then Vendor Balance Detail.
2. Select Customize Report, then go to the Display tab.
3. Select the desired date range for the report.
4. Go to the Filters tab, then select Account from the list.
5. From the Account drop-down, select the asset account created to track prepayment amounts.
6. Press OK.

You'll want to visit our Help Articles page for more tips and references for managing your business with QuickBooks Online.

Keep in touch if you need any more assistance with this, or if there's something else I can do for you. I've got your back. You have a good one.