A non well formed numeric value encountered - Laravel PHP

I have an application which has Purchase Return feature. I am not able to add more than 3 items in the purchase return. Going to the log it says,

A non well formed numeric value encountered

This happens in line number 96 which has the following code

$input_data['transaction_date'] = $this->productUtil->uf_date($input_data['transaction_date'], true);
$input_data['total_before_tax'] = $input_data['final_total'] - $input_data['tax_amount'];

Looks like there is some numeric / string conversion issue. Couldnt figure out. Any suggestions.

via Chebli Mohamed