You didn’t read the guides or followed the guides or followed recommendations given by Lubos above.
You are entering the balances wrongly. The field you are entering the balances in is not the place to enter the balances. Businesses normally owe money to suppliers based on invoices they have received from suppliers. If something happened and now the supplier owes you money, enter that amount in the Supplier Available credit field.
Use invoices to record the opening balances or unpaid amounts you owe to your suppliers as explained in the guides. Make sure the invoice date is set before your official starting date. eg, if your financial year is 1/1/XXXX to 31/12/XXXX enter those invoices on 31/12 of the previous year.
When you enter a positive amount in the available balance in the supplier edit screen you are recording an over payment to your supplier (that is you are saying that the supplier owes you money). So if you record another payment to the supplier this amount is going to increase,
The Available balance field in the supplier edit screen is ONLY used if a supplier owes you money. For example: if you have overpaid an invoice or the supplier gives you a credit note on an invoice after you have paid it.
The only way to record a supplier debt that exists at the start date is to enter the invoices that represent this debt, but you must enter these prior start date invoices with a date prior to the start date.