It sounds like you’re perhaps importing history? I have actually got a YouTube video on why this is super hard to do and I recommend against it … but if I am wrong this is what I understand.
When you enter an opening balance you can always enter a transaction so it Debits and Credits the same account. So for an AP transaction you would create a Vendor Credit or Invoice (for example) where the expense account on the invoice is the Accounts Payable account for that vendor. Then you have it in the subledger but the G/L is net 0.
Now if you have open un-applied vendor payments prior to go-live, I suggest you apply them in the old system to the maximum amount possible.
If this is to create payments for past invoices you’ve loaded and posted, then I believe your application to the AP G/L account should relieve the AP aged amount owning but create no net change to cash or the AP open balance (which is what I think you want).
The AP open balance should remain unchanged - just run a TB and make sure it still matches the expected TB.
Ultimately you want the Trial Balance to match the old system on the go-live date, and the Aged AP to match the aged AP from the old system. If those two match you’re golden.