Will a credit always decrease a cash account?

A cash account will always be decreased by a credit, but a credit will not always decrease a cash account. The only time a credit decreases cash is when the company pays out cash, whether it's to purchase supplies, inventory, or pay wages etc.

Here is two examples of a credit in a transaction, one will decrease cash, the other will not.

Company X buys $1,000 in inventory from Company Y and pays CASH. The debit for this transaction will increase inventory, the credit will decrease cash since company X is paying cash for this transaction.

Using the same transaction however, changing Company X wants to purchase this inventory on "credit" the debit in this transaction as above will still increase inventory, however, since Company X has chosen to purchase this inventory on credit and not use cash and Accounts Payable will be set up and the credit will "increase" accounts payable.

Remember, Assets will "always" increase with a debit and decrease with a credit. Liabilities will "always" decrease with a debit and increase with a credit.