invoiceninja/app/models/Credit.php

<?php

class Credit extends EntityModel
{
    public function invoice()
    {
        return $this->belongsTo('Invoice')->withTrashed();
    }

    public function client()
    {
        return $this->belongsTo('Client')->withTrashed();
    }

    public function getName()
    {
        return '';
    }

    public function getEntityType()
    {
        return ENTITY_CREDIT;
    }

    public function apply($amount)
    {
        if ($amount > $this->balance) {
            $applied = $this->balance;
            $this->balance = 0;
        } else {
            $applied = $amount;
            $this->balance = $this->balance - $amount;
        }

        $this->save();

        return $applied;
    }
}

Credit::created(function ($credit) {
    Activity::createCredit($credit);
});

Credit::updating(function ($credit) {
    Activity::updateCredit($credit);
});

Credit::deleting(function ($credit) {
    Activity::archiveCredit($credit);
});

Credit::restoring(function ($credit) {
    Activity::restoreCredit($credit);
});