Hi,

In Odoo 13, the field payment_difference_handling is defined within the account.payment model, but while coming to Odoo 14, it is defined in the account.payment.register model.
So you have to change the model in which you are inheriting the field and adding new values into the selection list.

class AccountPaymentRegister(models.TransientModel):
    _inherit = 'account.payment.register'
    payment_difference_handling = usage = fields.Selection(selection_add=[('reconcile_multi_deduct', 'Mark invoice as fully paid (multi deduct)')],ondelete={'reconcile_multi_deduct': 'cascade'})

Hope it helps