Resolution

On the Main tab of the Contract, there is an Available Balance field. But this is the total Available balance of the contract, and transactions are entered against specific contract lines, which have their own available balance.

Navigate to the Accounts or Amounts tab
Right Click on the column header, and add the field Available Amount

This will display each specific Contract line's available amount, which can be lower than expected and lower than the Contract's total available amount. A transaction against this Contract Line cannot exceed the line's available amount.

Browser not supported