A setting for the maximum threshold for writing off discrepancies in the allocation screen

If, when allocating sales invoices and receipts, your receipt value is greater than your invoice value you will be given the option (when saving) to write the discrepancy off as a discount. This makes sense for minor discrepancies, such as a few pence, however, for larger discrepancies it is unlikely this option would be intentionally selected and if a user accidently selects this large amounts may be written off unless someone manually checks the account. Would it be possible to introduce an setting whereby this option only appears if the discrepancy is below a specified value, i.e. £100?