Hi dvscomponents,

The credit will impact the balance of the suppliers account as soon as you post it, regardless of the date of the transaction.

As a workaround you could create a dummy supplier and post the credits against this account, and then edit the supplier credit (change supplier name on drop-down) when you come to pay the balance. You can also stop credits automatically applying to open bills by turning off the setting in the accounts and settings > advanced > automation.