Is it permissible to add a small percentage to sales invoices to cover a financial deficit in the treasury, knowing that this increase will not affect the employer or the customer?

The treasurer is entrusted with what is under his hand, and he is not liable for what is damaged without transgression or negligence. Your hand over the money is a hand of trust, and you are not obliged to pay for the financial deficit unless negligence occurred on your part. It is not permissible for you to cover the deficit by increasing the invoices, as this increase is not yours but belongs to the employer.