Firm Credits require being offset with a corresponding Firm Invoice (PI)
If it's a good housekeeping exercise, and the credits are not actually due to be applied to actual invoices, to clear these Firm Credits from the supplier payment screen, you could post 1 Firm Invoice for the total value of the Firm Credits, then pay the supplier picking up all the Firm Credits and a single Firm Invoice which will net to 0.00.
To post a firm invoice:
Go to Purchase Ledger.
Select Firm Invoice.
Enter relevant details.
Post Batch.
To Pay the supplier:
Go to Purchase Ledger.
Select Pay Supplier.
Tick all items in the allocate column on the right-hand side of the posting screen.
Enter the Amount on the left-hand side of the screen as 0.00.
Client Post.