Let me help, @Pelita.

The above procedure applies to your scenario if you have created a check and used an expense account. Then, you can deposit the amount to the same account used.

Another way is to create a bill for the purchase. Since there's a change of $20 for the said purchase, you can create a bill for $80 instead of $100. Then, create a deposit of $20 under Accounts Payable with the name of the vendor.

Here' how:

1. Go to Vendors menu and select Enter Bills.
2. Enter the name of the vendor.
3. Under the Expense tab, select the expense account used and enter the $80 amount.
4. Click Save & Close when done.

Then, make a bank deposit for the $20 amount and select Accounts Payable. Make sure to enter the name of the vendor. 

Once done, go back and open the bill. Then click the Pay Bill option. Check the transactions showing on the Select Bills To Be Paid. Click the Pay Selected Bills tab.

That's it. if you have additional questions about this process, feel free to ask. I'll be here to help you further.