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);
});