Retainer/credits option for companies

I’ve been looking for a way to keep track of funds paid by a company as a retainer. This would be an ideal option instead of having to manually track all invoices paid by the company.

For example, If a company paid $5,000 upfront as a retainer, the option to either mark that in the system as a balance or to give them credits on their company account that could be used towards paying invoices. This would allow the realty company to keep track of how much they have left and allow the photography company to know when they are getting close to running out.