When the AP department receives the invoice, it records a $500 debit (liability) in the accounts payable field and a $500 credit to office supply expense (received items).
Most contracts are net 30 or 60 days out, this first transaction accounts for that.

The company then writes a check to pay the bill, so the accountant enters a $500 debit to the checking account and enters a credit for $500 in the accounts payable column.
Actual payment. debit on checking because you spent money.
Credit on AC payable because you reduced your Supply Expense (previously credited) from your previous transaction.

Accounts Payable is not an asset but liability which has credit balance. If we receive invoice mean our credit is gone up.  Ordered Supplies as expenses ( non-inventory), Expenses are debited/ a debit balance. 

Supplies (Expenses) Dr $500.m
Accounts Payable Cr $500.
(Supplies purchased on credit)

Accounts Payable Dr $500
Bank Account Cr. $500.

Bank has Debit balance and if we write a check it is going down and we credit it.

Still I am struggling on the entry suggested by Maxpayne and Investopia.
If you are right then what am I missing?

You're thinking opposite. You reduced your liability by paying from your account. That's why it's a debit. (similar to your regular bank transcations).

Ac payable is credit because you reduced liability.
(Similar to your credit card payments) look at those statements as a example.

Just realized you are stuck on credit purchase vs credit on statements. These are two diff things.

When you buy sth on credit this happens.
Supply Increases (literally)>>>crediting here means you're assigning a positive value monetary wise to that account. (you could technically sell the supply to generate cash)
Your Debt increases: Debit Account Payable because now you have a negative value on this account. (your went from 0 to -100 to pay that later)

2nd transcation.
you pay 100

now
you have to cancel that debt (not debit) increase by paying the due amount.

Bank account gets debited (reduce the cash on that account so debit)
Account payable gets Credited (you are paying money to bring the account from -100 to 0 now, this is increase in account value cash wise) so we credit 100.

If you look at these both transactions at the end:

you can see
you credited Supply AC
you debited your back account (This would be a straight transaction if you paid cash right away)

you brought ac payble from 0-100 on first transcation; then on the second paid +100 to bring back to 0.(cancelled that prev transcation)

Thank you, Max
I am going to open my book and clear the cloud.